Endometriosis Support Group
Endometriosis is a common medical condition where the tissue lining the uterus is found outside of the uterus, typically affecting other organs in the pelvis. The condition can lead to serious health problems, primarily pain and infertility. A major symptom of endometriosis is pain, mostly in the lower abdomen, lower back, and pelvic area.
like swankmama suggested, try different positions. cowgirl (me on top) tends to hurt less, probably because in that position the woman has more control over how deep the penetration is. also ask your bf to be a little more gentle depth wise.
lots of foreplay and lube to relax your muscles is good too.
also i'm sure this is the hardest thing ever to actually follow, but if it's only happened once maybe it was a freak thing and won't happen again. at least, you should think of it that way because being tense about whether you're going to be in pain is just going to make it worse.
If missionary is a non-painful one, you can make it feel even better by putting a pillow underneath your bottom/lower back, it creates an angle that increases the odds of your guy hitting your G-spot (making the mundane missionary more fun!) And the more foreplay the better (thats true even with women who dont have endo!)
I know its hard but try to relax, if you go into it thinking "its gunna hurt" odds are it will. Hope this helps! :-)