I just potted my citrus tree. It started dropping leaves slightly before this. A palm tree on the other side of the room I have has spider mites. I see no signs of infestation in the tree in question. It is cold where I live and it is by a drafty window and a heater.
Indoor Orange Tree Dropping Leaves
Certified GKH Gardening Expert
The leaf drop may be caused by low light or caused by the stress of repotting. If the citrus tree was recently brought indoors, it will be adjusting to the lower light levels, and this usually causes some leaf drop. Providing as much light as possible, avoiding overwatering, and preventing infestation with the spider mites will help your citrus.