Certified GKH Gardening Expert
It is, likely, stress related. Being near the window, it can still get a little cool. You may want to provide a little extra heat when it gets cold. They don't do well when the temps approach 50 and below. They will enjoy a little humidity, and slightly moist soil, but will easily be overwatered indoors. Be sure to let the soil dry out down to about 2 inches, thoroughly, between waterings.
This article will offer more information on the care of the plants: