unusually light spotting and cramping before period..

I posted this on the pregnancy boards, but it might fit more under this topic..

My boyfriend and I had sex a couple times last week and on Monday night this week. We tried again on Tuesday morning, but I started bleeding lightly and had to stop. We used a condom every time. Since then (it's now Friday), I've noticed brown spotting, which has a bad odor and I've had some light stomach cramps. My period is almost a week early, but it's usually extremely heavy, with really bad cramps. It's never been this light. I've also had a touch of diarrhea. I also have a large ovarian cyst that's soon being removed, but I haven't had any extra pain associated with it, and it hasn't caused me any problems before. I'm really nervous that maybe we weren't careful enough.. could I be pregnant, or is this most likely something else? Also, is it too early to see the doctor?