During pentrative sex, after just a few minutes 2-3, cramp starts in the right leg thigh of my girlfriend and it becomes harder and harder for her to continue sex and she asks me to to finish quickly or sometimes stop at all. She mostly likes missionary postion but have tried other positions as well to avoid cramp. And out of all those missionary is the one less painful and takes longer to start cramp than others. Also started using water based lube but still the same. It saddens me that i am unable to take her to the full pleasure of her orgasm because of her pain. She also takes some medicine and it helps to relieve the pain after sex but does not prevent during sex. Any idea or suggestion?