What Causes Brown Vaginal Bleeding And Headaches?

I am 31 and had unprotected sex. I have been really tired the last couple of weeks and get headaches every day. My last period was on February 16th and they usually last 6-7 days. This one was only 2-3 days. I was supposed to start my period yesterday and I had some brown blood but that is it. Nothing today, but I feel like I have cramps. Any thoughts on what could be causing this?