No body knows why could be causing this?
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.
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.