The container growers here will undoubtedly enlighten you further; but you have already done most of the damage you can do. The potting medium is not so good for indoor use; you repotted it at the wrong time; so it has been given a double shock (along with moving indoors). Changing light, humidity, temperature all at the same time makes the tree think it has to change its habits to survive (anthrpomorphizing, I know). The tree will drop most of its leaves to replace them with leaves more efficient for its new environment. That is compounded by the flowers; I would remove them immediatley along with the fruit, as the tree will hold onto the flowers to the detriment of everything. As the tree gets bigger, it can survive these issues easier; because it will have more carbohydrates stored in trunk and limbs.
That's the idea of what has happened; as to what to do about it, I will leave to the container gurus; my limited knowledge is only about inground trees.

news leaves dropping doesnt sound good.

how moist does the medium stay? you can check with a bamboo skewer. generally all purpose mix stays too wet for too long for citrus. you may want to put an oscillating fan in the room, but not too close to the tree, you just want to add some air circulation.

how far are the grow lights, what wattage, how many lumens? generally for citrus and other high light requirement plants you want the lights very close to the plant.

once the temps drop, citrus fruits will not get any bigger, they'll usually change colors, yellow for lemons. pick the lemon now. it would probably be a good idea to pick off the blooms too.

Thanks for the replies!

The soil stays relatively moist for the first couple of days, but it doesn't seem to be overly moist (soil doesn't adhere to the bamboo stick) after a 3-4 days. The water drains readily out of the bottom of the container...it seems to be fully drained within 30 minutes or so, with very small amounts draining over the next few hours.

I'm not sure on the wattage/lumens on the grow light. It was the large, single bulb light from Home Depot. It's about 3 feet away from the tree, and angled at 45 degrees towards the tree, which I rotate periodically.

I have picked the immature fruit and removed all the flowers from the tree. There is no air circulation, I'll add a small fan and see if that helps. Should I have the fan blow over a bucket of water, increase the humidity?

Thanks again for the help!

Why dont you have this tree outside? Although your temps are probably cooler outside then in, if you arent seeing lows close to freezing your tree will be fine. It may not grow much but the complications that come with trying to push growth inside wouldnt be worth it to me if I lived in your zone. Here we can have lows in the high 20s almost every night till March so mine have to go in the greenhouse.

I would leave it out and only bring it in when your temps are consistently(several hours) in the mid to high 30's. It will be much happier outside.

mike

Makes sense, I'll do just that! Thanks for the advice.

I would think if you have any hope of saving a small tree that has been so seriously shocked, that you would leave it where it is and give it intensive care until Spring, when you can put it outside again and later re-pot it.
I repeat, change of light, change of temperature, change of humidity at the same time is quite likely to kill or nearly kill a quite young tree; it does not have the stored carbohydrates necessary to recover easily.