What's going on, am I pregnant?

I had sex on June 18, 2011. My projected date to start my period was June 21, 2011. I still haven't gotten my period yet. I took a pregnancy test on June 23, 2011 and I was not pregnant. One hour after sex, I began to spot, right away. This spotting lasted for seven days, it was heavier on some days and then got little. I'm experiencing some cramps, very minimal and my nipples are a little tender. They don't hurt, they're just kinda itchy and sensitive. But they don't feel sore or hurt to touch at all. For the most part, I feel fine. What's going on?