No trying to conceive. But I'm questioning this. Pics included!

Lori
I am on birth control first of all. I had my last period on April 17. I started taking antibiotics the week later, I did have unprotected sex and I also ovulated on May 2 and I had unprotected sex that day and was on antibiotics. I am having brown discharge rather than my period. I had a little pinkish red blood but nothing major. My period should have came on the 14th and today is the 16th no blood, still just brown spotting. I have taken 5 pregnancy test this month and all negative. I took one this morning after being two days late and it said negative. I am really worried because I am birth control so my periods are regulated. I am also having very light cramps and I never have cramps. Can someone please tell me what's going on?