Same thing has happened to me - I had to call my dr's office to ask if it could be my period and it was. It has almost stopped - been going for 7 days so far and changed pills halfway through. But my dr. said I could have some spotting when working out, so like GRose said it could just be that or becoming more active.
How long has this been going on? It could just be your cycle trying to regulate itself. If you have started exercising recently it could be due to that. I know for a while when I started working out again I would spot a little.
Maybe just call your OB and mention it to him, he might want you to come in and get checked out. Good luck and congrats!