OK when I lay down on my stomach it hurts very bad.. I haven't seen my period for like 3 months. I know I'm not pregnant so what do you tjink it is

What did your doctor say? Did he/she do a pelvic exam, X-rays, an ultrasound?

You need to go back to your doctor and tell him/her what's going on. No one can give you a diagnosis over the internet, not even a doctor. Please make an appointment right away.

Good luck.

Does it feel like you are laying on a basketball or something like that. You could have a cyst. Like others have said go to the doctor.