Sharp pains from rectum to vagina, periods, endometriosis! help please!?

I am 16 and I have had my periods for just over four years. They have never been perfectly regular, they used to be usually 4-10 days off (off of 28 days) but now I think they are becoming more regular. However I can't be 100% per cent sure because I have thought they were getting regular before and they didn't. Once when I was 14 I suffered my first bout of debilitating cramps. I felt light headed, nauseated, like I couldn't move. I had to go to the sick bay and just lie down with a bucket until it went away. Since then this has happened a few more times. It happens about every second period. It means I can't go to school or go out or to work. Sometimes if I take over the counter pain medication with Ibuprofen in it, the pain will lessen. If I don't, the pain usually lasts for just the first day of my period, and I have diarrhoea. My periods have been increasingly heavy. They usually only last 3-4 days but the first 1-3 days are very heavy. Even with the heaviest pad I can find (overnight super, maternity equivalent) I have to get up and change them. I have read that these symptoms are abnormal but I have been to the doctor for irregular periods twice now and she keeps saying nothing is wrong. I feel like such a hypochondriac for wanting to go back. I also suffer from sharp pains from my anus to vagina around the time of my period. They are so painful I cannot move, I just have to bite a pillow or something and wait for it to pass. They usually only last a few seconds. I did some research and a lot of people who have them also have endometriosis. I'm getting suspicious! what should I do?