Needing advice.

Start of cycle on the 28th
Ended on the 31st
BD every other day until the 11th 
My most  OV days were 7-10th and on some days 7-9 BD every day
Thursday 6 days past Ov I started spotting.... Nothing major so light didn't even need a pad.  Tested every other day and still bfn. I'm always regular and on time never late and never spotted before. Any ideas?? Anything would help.    Thanks