I lost my balance standing on an air mattress and when I fell my my right arm went behind my head and now I cant hardly lift it and I have pain in my?

Injury to Arm. You have either bruised your arm muscles or may have broken one of the bones in the arm. The only way to know what to do is either go see a physician or let us know how your arm looks on the surface? color? ?swelling? Can your move(bend and roll your wrist? What pain medication are you taking? If you send these details, we can offer you further guidance.
TrainingForOlympics? That sounds like a mechanism that could have led to your shoulder being partially or totally dislocated. Maybe it went back in place but you should have it checked out ASAP.Hopefully it's just a minor injury, but maybe not.Put ice on it now, take some Motrin and then get to an Urgent Care or ER.Standing on an air mattress,huh?Training for the Jackass Olympics? [Wikipedia: Jackass TV Series]