When that happened to me, and I'm not saying for sure this is exactly your situation, it was implantation cramping and bleeding. I thought it was my period because some of the cramping was really painful but it was implantation. It happened with both my pregnancies but to a lesser extent with my second. If you are regular enough to know where you are in your cycle, that will usually be what tells you what is going on.
at 10:45 AM on Nov. 14, 2010