It could be fibroids, which may need an ultrasound to detect. You should find a new doctor quickly to avoid becoming anemic. From what I understand, the rule for bleeding that needs treatment is soaking through a pad or tampon every 2 hours.
I've been sick with other conditions for a long time, and I've learned that if you keep going to doctors who don't take you seriously, you never get things fixed. A doctor who doesn't care will never care.
My new doctors have diagnosed me with several conditions and I'm starting to feel better. Good luck - you will feel better once you find a better doctor. Asking friends, family, and teachers for referrals is the best way to do this.