Tightening up during sex?

I've tried looking this up on google and such, but couldn't find the right answer, so now I'm turning to Glow.. There has been some issues while I've been having sex with my man where my vagina tightens up so much that it starts to hurt him and sometimes he can't even enter me because it's so tight. I have no control over it and he thinks that I just need to relax, but I just can't. Does anyone know what I can do to fix this? Does anyone know what this even is? I'm not supposed to hurt him while we're in the act so I would love to fix this..