Light to moderate exercise

I just began taking a bellydancing and yoga class. I love both classes but there is a problem that still arises in my mind: What if I lose weight doing these activities? Before starting my daily regiment I was not able to gain weight, now in two short months I went from 110 to 121. I want to continue doing the classes but I want to know how i can mantain or gain while I am taking them. What sort of calorie intake should be my goal for the day? Are there certain foods I should eat before and after each session?