LATE PERIOD..NEG PREG TEST..POSSIBLE STILL PREG?

I'm 22 years old and I was supposed to start my period on June 11th and I still havent started yet. I used to be irregular and very heavy whenever I did start my period but ever since I got with my boyfriend (january this yr) I became regular (possibly because I'm sexually active on a reg basis with him), I have it for 5 days with a normal flow and I start on time every month. Except for now. Me and my boyfriend do have unprotected sex quite often and he usually pulls out. There has been about 5-10 times he has ejaculated in me but I lost tract around the times when he did. I've taken 3 preg tests and they came out negative. I took one last week and the other two each week before that. I have been having cramps and sore breasts for the past month on and off and I think im about to start my period but nothing. I also called my gyna and the nurse made it seem like i had nothing to worry about I also asked if I can take a blood test or something to have a more accurate answer to everything and she said just take another home preg test. (kinda pissed me off..whatever!). However, I did get a promotion here at work and have been a bit stressed out. I know stress can cause a late and/or missed period. Anyones opinion or comments are greatly appreciated :-)