getting pregnant

I am on birth control. It called Depo Provos. so, my boyfriend and I had sex every weekend.. we are sexually active a lot. but I don't know why my back is in pain a lot.. I know birth control can happen to get pregnant. I found out pregnant test was negative.. I really want to have baby with him. I don't know why my stomach and back are in pain sometimes. It bother me. I didn't sleep good a lot. help! Please