i used to have that problem alot of years ago, i went to see my GP and he told me it was just the muscles from having sex tightening n relaxing, due to my partner being on the large side it made me a bit sore and it would settle after a while once i got used to it. Unfortunatly there was nothing that could be done. And after a while it did settle and i no longer have the problem.
i did find that it was worse if the sex was a vbit rough and deep penetration so try just being gentle for a while and build it up to where you are comfy.
I hope this puts yr mind to rest if you still are worried maybe go have a word with yr doc for reasurrence