use your heels to bouce, and try stand straight while you do it, dont bouse 'through' your back, so when you land your back doesnt curve, and jar your spine/neck. this can lead to back injuries (and didnt help my back but hey!)
otherwise, it may be because you have used muscles you dont normally use so they hurt because they are streching, which happened to me when i first got my trampoline
Mini Ellis xxx