Kickboxing class

Just got back home from the local YMCA where I have committed myself to kickboxing class 3 times a week and step aerobics 1 day a week. I tell you what, kickboxing class kicks butt. The only thing I hate about kickboxing class is that we do too many jumping jacks. Jumping jacks = pee in the pants! The other problem I have is that my right knee is starting to bother me when I am jumping all around the aerobics room, it feels like it is going to give out on me every time I jump around on it! I never had this problem before, I am hoping that it will go away when I lose some of this weight. In the meantime I am modifying the workout, I don't know how much pee my pad can hold! and I don't want to be the girl that falls on her arse in the middle class. That would be embarrassing!