Hi

I am 26 and have experienced the same issues.
I too only get it on one side, and occurs during intercouse. I have not had this with previous partners, and I know he is not too big or hitting the bone.

I have just been to my doctors and she's doing an STD check as a pocess of elimination. When it comes back clear she wants to send me for scan, but she mentioned one of the causes could be a muscle spasms, so that it is the inability for a particular muscle to relax.

I think psychological reasons can play a bit part of these sorts of things, I know I find myself being a little disengaged sometimes and for me I think maybe that is the reason why I can relax .but am unsure.
When I find out more from my doctor I shall let you know.

I am so greatful you put this question up as I felt as though I was the only one experiencing this and it is comforting to know that other women fell the same.

Hope this helps