I keep it in my office which get pretty warm and stuffy when there isn't any air conditioning
It sounds like it may be too dry. If the pot has a drainage hole (it should) and the soil feels dry to the touch, go ahead and give it a good soaking. Misting is good but no substitue for proper watering.
indoor air is usually very dry as well as being warm, not good for most plants. Air conditioning also dries the air. Growing bonsai indoors is more difficult than keeping one outside. The first step, to my way of thnking, would be to get it outside in shelterd location and try to stablize it.