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Shivadiva..Wo!
When you took your lemon tree outside last June, did you have it in the same area it was in previous years?
I'm curious if by chance it received less light than the past? You mentioned medium light..not full sun? Citrus need direct sun, especially variegated..

Why did you trim roots? Trim roots than transplant in, 'possibly' a larger container?
How much space is between rootball and inside container?
The reason I'm asking is, in many cases, some ppl have a tendency to overwater. Large pots = more soil, more soil = more water, and chances are if overwatered, soils remain continuosly wet, plant will die: Overwatering is #1 killer. Symptoms include yellowing and/or 'leaf drop,' white fuzzy mold around trunk or atop soil. Mushrooms.

You said, 'I did not flush the pot since I didn't know about that. I used Cactus soil for that.' What does that mean? lol..

Did you use other mediums with cactus soil.
Cactus soil is well-draining, but I don't know how it does without adding other mediums.
People use soil, soil-less and combined mixes. Although I use Perlite, sand and sphagnum peat, (soil-less) IMO, citrus need 'fresh soil' for nutrients. Some will disagree, it's a matter of choice.
Cactus soil differs depending on brand..some are well-draining, others hold water. Why did you choose Cactus soil?

What size is its new container? Measure diameter, from one side straight across to the other.

As long as stems are alive, let them be..live branches are flexible. They bend without snapping. To check whether or not a stem is alive, scrape a little outer bark. If the inside is green, it's alive, if brown/tan and breaks, it's a goner. Clip off dead branches, and remove any damanged leaves.

You did the right thing withholding fertilizer. Since winter's approaching there's no need to add any now.

If it were my tree this is what I'd do.

Place in the brightest south or west window. As soon as sun sets, on would go artificial light.
Allow soil to dry between waterings. To check, I'd insert finger or probe deep in soil. If finger or probe is wet/moist, I'd wait a few days and retest.
Once a week I'd carry to sink/shower and hose foliage, plus daily misting.
Keep away from heating sorce, especially vents.
Preferable temps above 50, below 68F.
Once a month I'd add Superthive.
If air is dry amd stuffy, I'd invest in a humdifer, and turn on a rotating fan a few hours a day. Air from fan should't be aimed on plant.
Dead branches and leaves removed.
Come longer days, a well-dilluted dose of Citrus/Azalea fertlizer would be given. Or Miracle Grow.

Repotting can cause shock, too.
Insect inpsection should be done once a week, especially in winter.
Good luck, Toni

Shivadiva
First what city state are you in?
You definitely have a weak root system. Now there could be many reasons why. So many times on this site people are warned of over-watering their citrus. It is correct that over-watering is not good for citrus but with a good draining soil over-watering is tough to do but under-watering can be the real problem.

Thanks for the comments.

I used cactus soil because it's what I had -- fast draining organic material with some grit. (Came from my mom, perhaps was 2 mixed bags of different stuff, but looked a lot like what was in the pot). I have bought a bag of 'citrus' soil and was going to use that to repot.

I trimmed some roots (but not much) because I read that on on of the sites that sells a lot of dwarf citrus trees, about repotting.

The pot flushing is another thing they recommend, to take out the salts. I see hydrogen peroxide on here and that's interesting, I had never heard of that, but I am very leery about doing something like that to a plant in stress.

It's unlikely that it's overwatered. That would be hard to believe since we had a drought for 8 weeks July-Aug and all I did was wash off the foliage with the hose about every week, to moisten the leaves (since they look dry). As I said, it didn't like full sun, wilted, and perked up in the medium shade situation. Very strange.

I'll definitely put it in my best south window. I'll have to tent it in a dry cleaning bag with hoes in it since I have 2 plant-eating kitties (unless tenting it in clear plastic is a very bad thing).

http://www.ubcbotanicalgarden.org/forums/showthread.php?t=21800
http://www.ubcbotanicalgarden.org/forums/showthread.php?t=29974

Citrus, the reason I mentioned overwatering is, Shiva used Cactus soil to pot his/her citrus. Most cactus soils are well-draining, others aren't. It depends on the brand. My succulents are potted in Cactus Soil yet still need Perlite.

Shiva, what are the two websites for? What is it you want us to see? lol

If your soil is as well-draining as you claim, and the only water your plant is getting is from rain, it's probably underwatered. That could be the reason some of its leaves are drying out.

Did you recently buy Citrus soil? Is it Miracle Grow brand?

Flushing is mainly done to rid salts due to fertlizers. If you suspect your lemon was overfertilized, or if you accidently added too much/too often, flushing was a good idea.
I doubt your lemon isn't a sun-lover, but if it was previously growing in a dark spot, then suddenly placed in direct sun, OR, if underwatered, leaves will limp or drop.

Plastic bags should be okay, but make sure you slit vents for air circulation. Being enclosed in plastic will hike up humidity, but citrus need fresh air, too.
Bad kitty's..lol..One way to keep cats away from plants is by sprinkling Cayenne Pepper around the rim of a pot, direcly on soil. Cats usually detect its scent a few feet away, so instead of devouring leaves, they should ignore the plant. Pepper will not harm your cats..I'd never, ever tell someone to use something that would harm pets/kids. Even if your cats decided they liked the taste and nibbled, other than wanting a drink of water, pepper wouldn't do ANY damage. I have pets, too. They are a big part of our family.
Good luck, Shiva...Toni

most cactus soils plus grit ='s well draining, therefore plant/s require more water than one potted in a heavier mix. It's possible, if not probable your citrus is potted in totally soil-less mix..if this is the case, water more, don't depend on rain..Unless you live in an area rain falls daily.

This looks to me like a clear case of under-watering.
If you really didn't water it for eight weeks of drought, then you have a dried lemon plant!
There are so many warnings about overwatering citrus that people often go to the other extreme.
When you trimmed the roots, you reduced the plants ability to gather and transport water to the leaves - so they dropped. If you trim roots you must also reduce the leaf canopy to match.
I guess the soil is bone dry in the centre. When you water it will just run through the very free draining cactus mix. Citrus mustn't be permanently soggy - but they aren't cacti!
I would soak the plant in a bucket of very slightly warm water (around 80F) for several hours or overnight. Then let it drain out completely and re-water whenever the surface becomes dry. In summer sun that could be every day.