How possible is it that I am pregnant right now?

So recently I have noticed many "pregnancy symtoms" occurring. I know that there are many causes for some of these symptoms but it does seem very suspicious. I have had very tender breasts that are recently beginning to stretch the limits of my bra. I feel nauseous throughout the day and extremely dizzy (especially after a few hours at work). I am even beginning to gain a small round belly (I am pretty thin normally). Others include excessive amounts of discharge, cramping, and missed periods. Also, about two months ago, there was an "incident" during which I could have gotten pregnant because stupidly, no birth control was used. Here is the thing though, I have taken 2 home pregnancy tests in the past week and both were negative. Could this be the result of some other ailment or should I trust my instincts, which are telling me that I am very much pregnant. Some advice would be very appreciated ladies/doctors/whoever. =] Thanks!