I'm 18-years-old, I Have Only Been Sexually Active With One

I'm 18-years-old, I have only been sexually active with one person, my boyfriend of almost two years. It is suspected that I have endometriosis. To treat my endometriosis, my doctor put me on Birth Control to stop My from bleeding, but I bled for two weeks straight on it, so I stopped it...that's been a month ago. My period is almost two weeks late, and I have had intense stomach cramps for about a week now. My breasts are tender and heavy, and I have been having to urinate every ten to twenty minutes and it's an urge is very hard for me to control. But the thing is, every pregnancy test I take, including one from the hospital, have been negative. What could be going on? Could I still be pregnant?