Terrible Periods

I'm 17 and have been having excruciating, long periods since I was 12. I bleed anywhere from 8 to 20 days heavily and afterwards get pain in my ovaries so badly that I have to limp (usually just on one side) I always miss school and work due to my periods. I'm always in terrible pain on my periods and can barely move. My doctor says I'm probably just exaggerating and when I got checked for endometriosis they said everything looked normal. Even pain pills don't help at this point. What can I do?