What Are My Chances??!?

I was under the impression that people don't get their periods when they're pregnant, but after reading these posts, i'm terrified. I (stupidly) had unprotected sex (w/o ejaculation) a little over a week ago, and I began my period a few days early (i get my period every 23--26 days, and it's never on a specific date, but it's always there), and it began as a light, ruddy brown discharge, and has since become a heavier, bright, bright red (with minimal clotting). Despite the fact that it's reasonably heavy, it isn't as heavy as it usually is, and since last night, i've been worrying myself to death that it may be implantation bleeding and that I may be pregnant. Aside from having a lighter, somewhat earlier period, I haven't had any symptoms--no nausea, bloating, or breast tenderness/swelling. I did do strenuous exercise yesterday, have been under stress, and haven't been getting much sleep lately. But my periods are usually pretty consistent.

I took the htp today and the results said "negative." but i've heard that the htps aren't always accurate. I'm soooo frightened, and I hope that this is a false alarm. What are the chances that i'm actually pregnant and what should I do?!??