intense pain from sex

okay so, ever since I've been with my boyfriend, I have had trouble with sex. When we do a certain position it will feel good, I don't tense up or anything, but sometimes he will hit something like on the right side on my uterus and it causes extreme pain. I've had my breath knocked out of me and I've even cried before. it's like menstrual cramps x10 but only for a few seconds. Does anyone know what this is?