play sitting without the strap in a position where the bass doesn't slide off till you can get to a doctor and find what the problem is... IF your doctor can sort that out. i'm not a doctor, but there could be more than one thing that could cause this problem. some could be improved with certain exercises, but some could be made worse. but at a guess, your bass could be too heavy for you or your strap too hard. btw, does your strap have any harder or protruding areas where it rests on your shoulder? maybe a thick seam?