Don't let him go all the way inside of you he could shift your womb and that is very serious.When you're having sex let him find a sweet spot upinside you that you like aswell that is'nt all the way inside you.
My first thought would be endometreosis, but I would think it would hurt more than just the once. Have you talked to your OB/GYN? That's where I'd start. Sorry, I know that isn't very helpful. I just didn't want you to think that everyone was ignoring you. Good Luck.
No body knows why could be causing this?