So on day 11 of my cycle I had a little bit of pink spotting (just when I wiped) but this has happened in the past so I ignored it. Then on day 13 and 15 I had more pink/brown bleeding (enough to fill a panty liner.) The blood was watery, and mostly light pink. On day 16 and 17 I had mostly brown bleeding, not as much though. I went to the ER on day 15 and they tested for STD's and the culture came back negative. Other than that they provided me with no answers. 

My typical cycles are 23-31 days. I have had unprotected sex this month, when he did finish inside of me but I was on the second day of my period so I highly doubt I'm pregnant. So my question is: what could be happening? Has anyone experienced this before and if so, what did you find that it was?