My Doctor Thinks I Have Endometriosis

Im 17 years old and I have been having my periods since 4th grade. Before I started having my periods, I would have HORRIBLE cramps at night just a few days before Id start. Theyd last all night and nothing would help them. And then they stopped until about 8th grade. Id still have the pain, but not nearly as bad. Now I have cramps that basically make me want to shoot myself.
I have gone through midol, motrin, naproxem and codeine. None of them worked for very long, if at all. And now Im on Vicodin and Ibuprofen. Even Birth Control Pills wont help me. Not even too much with the flow of my period. My cramps still last most of my period too.
I went to the doctor the other day and she said she thinks I might have Endometriosis, considering that we have tried everything above. She sent me in for an ultrasound, but it came back normal. I have read that not all ultrasounds for endometriosis show anything unless its severe usually...
Im wondering if Im just an abnormal case of severe cramps or that I really could have this.
I have never really read anything about anything else that could affect me in this way...
Should I rule out endometriosis or not?

I guess Im just a little anxious right now...
Because I already have to wait three-four more weeks before my gynecologist appointment and Im also going to be on my period again that week...
Any advice on this?