What Can Cause Delayed Periods?

I am nineteen years old, and weigh about 220 pounds. My boyfriend and I fooled around (not sex, but the penis and vagina were touching) , but I had just gotten off my period the previous day. I'm on birth control, and this happened March 23, 2011. I took a pregnancy test about six days before what would have been my first missed period. It was negative. Then, a few days later, I was bleeding but it wasn't a regular flow, just red and brown discharge. Now, I have heartburn and when I eat, I get full really quick, but then feel hungry again soon after. I've had no other pregnancy symptoms, but is it possible that I am pregnant?