Was the tree recently moved? Is it near a heater? The indoor air could be too dry - desert dry. What zone are you in? A blast of cold air could also do it. Imagine sitting poolside in sunny Florida sipping a Pineapple Margarita. This is where your lemon tree wants to be.

I agree with Bathgate. they resent draughts and this sounds typical of that. Have you altered the watering regime?

How much sun is it getting? Indoors, Citrus need a lot of light.