After Sex, I Get Intense Pain Running From My Backside to My Knees

Dear Betty,
So happy to have found your site! Am having fun and learning a lot...
Have been having an... Issue... And can't find helpful info anywhere. About three to five minutes after sex, i get intense pain from my lower butt to the back of my knees. It's kind of like muscle cramps, but different. Deeper, and no amount of massage helps. Sitting or lying down helps, and I usually fall asleep anyway, but if I have sex during the day I have this pain for hours and can hardly walk.