Had Unprotected Sex. Missed The Pill. Had Brownish Discharge. Now Have Cramps, Light Period. Causes?

Hi, I had a question. My last period was April 13th. It's usually like clockwork, always on time. Well yesterday was the 21st which is quite late for me. I did attempt to be in birth control. I took four pills of lutera beginning on April 23rd to April 27thand gave up because I had decided to later get implanon when I go to my gynecologist at the end if the month. On may 4th there was this brownish discharge and a bit of blood, but only for three days. It was extremely light so I didn't consider it an early period. My periods normally begin very heavy(changing a tampon every four hours) and work their way to being extremely light. My boyfriend and I had sex several times that month, All unprotected. Yesterday I had cramps, bad ones, but a very light period. Why is this?