Perhaps a root problem.

Tell us more about their conditions and what you do for them. How moist/dry do you keep the soil? And more.

Your cultural habits if you would please?

Have you made any major changes in your care? Any major enviromental change?

RE: Conditions.
I started keeping both of them in my back room (an addition) with two huge skylights so it is filled with natural light. I did put them in larger pots this summer with new soil; they grew SO much, almost doubling in size. I have noticed they need more water this year than in prior years, maybe because of their size? I do let the soil get pretty dry before watering them again. The pots do drain well, ummm. This is my first attempt at Citrus Trees do they like humid conditions? What is the best soil for these kinds of trees? Would it be safe to change their soil during winter? I just tossed them in some miracle grow potting soil, but that was last spring. I have moved the other lemon tree to a second floor bedroom; it gets great morning sunlight and is doing well. Any advice you can give me would be great, if you need more pictures, I can provide them.

what's your location? I don't know about growing lemons indoor but lemons need a lot, and I mean a LOT of direct sunlight. a skylight or a little morning sunlight is just not going to cut it. You might have to supplement with some artificial light if you can't get them out in full sun.
Lemons also like to be fertilized well. I would say to fertilize it at least 4 times a year. As long as you don't overdo it and burn the roots, they can take quite a bit of fertilizer and tend to be heavy feeders.
another thing is that citrus generally hate to have their roots disturbed, so you don't want to do too much re-potting and messing with the roots. They do naturally lose some leaves sometimes and from your photos it doesn't seem to me like it's that horrible of a problem.

Bingo!

Greendesert gave you very good advice..In fact there is nothing to add but this:

Because your plant is not getting the amount of sun it wants, it is going to drop leaves, change color, and drive you into a panic mode..
It is just a natural process.
Just watch that you do not water until dry as you say, and it will come back when the days get longer, as it's need for more fertilizer and water grow. It might even blossom like crazy.

The only time I would worry about it, is if the branches start to die back. If so, then you need to come back for help again..

Mike..:-)