Long story, prepare yourselves.

My last menstrual cycle began on the 17th of October and ended on the 21st of October, since then I have had no period. I am usually a high-stressed person but I have been no more stressed the last month than any before that therefor I don't see why my period would be stress-delayed 18 days (almost 3 weeks).

For the last six + years of my life (I am currently 21 years old) I have had extreme signs and symptoms of Endometriosis. My family doctor as well as physicians who specify in Women's Health believe I have endometriosis. While it was a personal choice of mine to endure the pain on a monthly basis with this potential problem (I have not agreed to surgary, ex ex due to lack of funds), my body has a regular schedule at which it happens. On the 17th (Maybe a day before or after, it varies) I am doubled over in such extreme pain accompanied by life-sucking cramping that accommodates diarrhea I begin my period once this pain starts. (The pain doesn't usually last more than 5-6 hours but it is incredibly draining).

I had that painful day and yet no period ever occurred. I have taken two at home pregnancy tests in the last week and both of them have been negative. My period has never been off track more than 72 hours in 10 years. I have called my family physician's nurse who has suggested that if what I say is correct I would be around 1 month pregnant. She suggested to make a doctors appointment but I don't understand the point of making a doctors appointment (so they can use the same pregnancy tests I do) to have another negative result. If I am not in fact pregnant, what on earth could delay my menstrual cycle the way it is currently?

I do have various signs of early pregnancy (went through them via telephone with my doctors nurse) but the tests are telling me no.

What should be my next step?
Is there a possibility I am pregnant?
I'm unsure and frightened at this point. I've never had a period I've gone through the bad pain / month and not haven my period. Every day is a burden as I feel even more lost.