I've been haivg really bad uterus pain while having sex. the pain gets so bad that we have to stop. can you tell me why this happens?

Painful intercourse: Deep in the pelvis could be a number of things, all of which would dictate that you get seen soon. Highest priority is to rule out an infection of the cervix or uterus. It may be a bladder infection or a few other things but please get some attention. Good luck.

Pelvic bones: Pelvic pain in late teenager during sex likely musculoskeletal. Legs drawn up & back while having sex supine is, for women, arousal of pelvic mechanical stress & painful ligament strain, especially of pubis symphysis; interpreted as "uterine" pain. See an osteopath for proper diagnosis & therapeutic advisories. Do you have joint hypermobility syndrome? See brighton criteria at wiki to decide.