Answer: If you've recently moved your tree indoors, it may just be reacting to the change in environment. Once it adjusts, it should produce healthy new leaves. When moving a plant from indoors to out or from outdoors in, take time to help it adjust gradually. I take my indoor plants out for a few hours each day, bringing them back indoors overnight. By allowing them to stay out for longer and longer periods of time each day, they'll be ready to remain out all night long in about a week's time. To bring them back indoors, just reverse the procedure, beginning when overnight temperatures reach about 50F.
If you gradually acclimated your tree to the indoors and don't think that's the problem, or it lives indoors all year around, here's another suggestion: 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. I hope this helps!
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