What Causes Dark Periods,back Pain And Breast Tenderness Despite Having Negative UPT?

I had a normal period Feb 28-March 4,March 9 I used the pull out method and was also ovulating March 23-26 I had a strange period that was dark Brown April7 used pull out again, ovulation on April 9.For the last week I ve had a discharge, backache, cramps and now my breasts are tender. I took a hpt today and it was neg.