Being on top

It hurts when im on top like hes stabbing me sharp pains ill cramp after. I can only do it for about 3 seconds then we have to switch positions. i am in pelvic floor therapy she said my muscles further up are tight the end goal is not to have pain during sex anyone have any suggestions? It just feels kinda defeating when he ask me to get up there or i see men commenting on facebook about how they want their women on top. I want to be able to do that for my guy and feel like not as good or kind of a let down now hes not said that, thats just how i feel because i know he really wants me on top i want to provide that service for him