Answer: It's difficult to diagnose a plant problem long distance, but we'll do our best to guide you in the right direction so you can determine the cause of the yellowing leaves. Although citrus trees are evergreen, plants do eventually lose their oldest leaves so if the affected leaves are towards the inside of the tree or lower down on the tree, the yellowing leaves may very well be a normal, natural thing.
On the other hand, if it's a sudden occurrence after a long spell of good health, look for spider mites, whiteflies, or scale on the plant. If you find some critters, submit another question to the Q&A page for a solution.
Overwatering can cause leaf curl/drop. In the dormant season, wait until the soil dries out about 1/4" deep before watering and keep the tree in a cool (50-60F), brightly-lit place. If the pot is on a tray, do not allow water to collect so the roots are soaking all day. On the flip side, if the humidity is lower than 50%, citrus also suffers. Mist the tree daily or place the pot on a bed of stones in water. The evaporating water will increase the humidity, and the stones keep the pot elevated out of the water.
Citrus need lots of bright light, though they do benefit from filtered light at mid-day. During the growing season, keep soil evenly moist, and take care not to overwater.
Hope this information helps you determine just what the problem might be. Good luck with your tree!
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