I'm so paranoid!

About me:
I'm Brooke, 15 years old. I had sex for the first time about five months ago on January 23, 2010. It was protected with a condom. I don't think it broke, but I'm not sure. & it definitely didn't come off. It lasted about 7 minutes or so. That was it, I haven't done it since then.

Lately, for the past few days, I have felt really fat and really freaked out that I might be pregnant. I'm about average in weight for my age. I weigh 122 now, but in January, I weighed 118. Earlier in the year, I played volleyball in athletics, and that helped me keep weight off. I stopped in December and being eating a lot after that. Being fat runs in my family, on both sides. I'm not sure if I'm just paranoid or if I really am pregnant. & I'm not sure if I look it either. In fact, I have no idea what to think.

I have had a period* four times since then. But, I have heard you can have your period during pregnancy. So, I'm not sure if it's really my period or if I am having coincidental vaginal bleeding.

Two days ago, I told my Mom about it because I was really worried. She was surprised to find out but was understanding. We went to get a pregnancy test at the store, & I took it. It said negative. I still feel worried. My Mom said there could be no way that I'm pregnant but I just feel fat like I am.

Please someone help me. I really don't know what to do. If you need more info, please ask.