What's going on?

Last week, beginning Thursday 12/10 through this week Monday 12/14, I had brown cm (sometimes almost black early on). AF was supposed to be here on Wednesday 12/16 and still isn't here. I've had a pretty consistent headache since the 10th and some nausea. I took a test this evening and it was a BFN. I called my doctor earlier this week and they said call back in a week if I don't get my period. Any ideas what could be going on? Any insight would be very much appreciated! I feel so confused.