Did you try to flush your soil since you have your tree (13 months)?
If not salts may have built up on your soil and you should get rid of it.

Your post sheds some light on the situation. I recently added soil to the pot, as a years worth of evaporation had removed a few inches of the top soil. When I added the soil I put in too much fertilizer (Miracle Grow Acid Loving Plant Food). I gave it a lot of water, however, the added soil was very very dry, and the water did not soak all the way through. I let it really dry out until the next watering. With the next watering, I really gave it a good soaking and the water started to drain. I suspect, that then, the added fertilizer started to mix throught the pot. Since then, the leaves began to turn yellow and fall off.

After your post, I looked carefully, and saw a white powdery substance on the top of the soil. This must be the salts that you are referring to.

I am afraid to flush it now, as it is very water heavy, as it has not gotten much sun lately.

Thanks

Citrus on the average should be flushed 4 times a year. You use at least 4 times the volume of your pot minimum. The more the better. It is just like taking a bath. If you take a bath you feel good.
If I were you I'd flushed it now or ASAP and then don't water until the first inch of soil becomes dry. It takes a lot of soaking perhaps weeks before the roots is affected by root rut.
It would also be nice if you know the pH of your tap water so you can adjust the way you fertilize your plant. Citrus optimum pH is 6.5

If you want to have a good healthy looking citrus just keep the between pH 6.0 and 7.0 (to admit nutrients to their system),well draining soil, 5-1-3 fertilizer (close to it in proportion) and sunlight. And don't water too much.

MY LEMON TREE LEAVES ARE FALLING OFF DUE TO THE RECENT FOST I HAVE NOW MOVED ALL THREE INDOORS WILL IT BE TO LATE /BRIAN

My 2 yr old lemon tree planted in a container is dropping off leaves that turned yellow. I fed it with a teaspoon of sulfate of ammonia in March. It has lots if blossom and has not fallen down. What can I do to prevent the yellowing and falling off leaves? Thank you.