Check the roots - are they rotting at all? Sounds like a root problem in my opinion. What is your watering regime and potting mix?

I already checked, and they are perfectly white, healthy as can be.
It has to be something else....I think it is weird. The rest of the whole plant is looking great. Even looking nice and green. Growing new growth too. It is the very very bottom of the bush. Only a couple of branches though the leaves seem to be turning hard right on the branch. I just touch one and it falls. Otherwise it will stay on and look like it's healthy. The oranges on that same branch are doing fine though. Call it paranormal experiences with my crazy plants. No one can even figure the pests I know have. Thanks though.
Can this happen from a one time underwatered moment, like last week?

I had the same problem with my calamondin that was planted in the ground and got heavy rain and floods.
Because the tree got weaker, it was hit with leafminers. That sounds to me like your problem with your other citrus.
I planted in the container with no hopes of surviving. I use Al's mix with turface and spray Spinosad.
Now is alive and healthy.
Google leafminers and compare to what you have, they sure are hard to get rid of and do a lot of damage on citrus and other crops as well.
The bad part is that you cannot see them until you see the damage.

How old are the leaves? Citrus leaves only live for 18-24 months, then they fall off. If they fall for any other reason, it is because the plant has become stressed about something. Assess what this may be and change accordingly...water issues? fertilizer? light? cold? temp change? humidity?

Mike, this never happened to my Calamondin, but I have a baby Mimosa tree doing the same thing.
I figured out the problem last wk. It's potted in soil less mix, (which is something I don't normally plant in.) The soil was drying too much..Like your Calmondin, leaves started dropping from the bottom. Texture was hard/crispy.

Shake your Calamondin so all loose foliage falls..Remove leaves that land on soil. Dead leaves attract insects. I believe you said the room is humid, in another forum.
Although humidity is important, so is fresh air. Since it's too cold to open a window, run a fan, preferably rotating. If you dont have a rotating fan, direct fan away from plants, so cold breeze doesn't aim at plants.
1-2 hours during the day will do the trick..

How dry do you allow soil to get? Toni

Toni, you hit it right on the nail!!!
Your right, just so happens that this particular plant is growing in soiless mix in which the particles of bark are a little to big and there is lots of air space. This is the first and only plant I planted in this batch of soil when I first started to learn how to make it. I know I made it incorrectly, because the particle sizes are too big. All my other soil I made, I used smaller particles of bark which has worked perfect for me. And now that I perfected it, I should repot thsi tree in spring into the perfected soiless that stays moist a little longer than a day at a time...lol
All my other plants in this perfected soiless mix can go for a 2-3 days under lots of light which I provide all winter, but this Calamondin tree is in a soil that I have to watch and wet everyday. I suppose to much air getting into the soil. A couple of times I forgot to water, and I think instead of the tree wilting, the leaves just go into shock and dry up hard on the branch because they are thicker older leaves. With this tree, you can never tell if it doesn't have enough water, because it doesn't wilt when too dry like my other trees! Since I have been watching this one more closely, and watering everyday now, I am not loosing anymore leaves. YES!!!
See, I guess we can hurt a tree from underwatering too...
:-) Thanks alot everyone for all your help and thoughts!