A sudden and protracted drop in temperature could cause this to happen - assuming you can't see anything on the plant, such as mealybug, red spider mite, etc. It has been very cold on and off during the last few weeks and perhaps the overnight temperatures wherever you keep it fell lower than they have before.

Check that the drainage holes in the pot aren't blocked as this would cause it to be waterlogged. Also, maybe repot it in spring.