I Was Suppose To Start My Period June 23 I

I was suppose to start my period June 23 I was 13 days late and today July 7 I woke up and was bleeding I also had an excruciating lower back and abdominal pain. I took a pregnancy test prior to this since I was late but it came back negative. I'm always on time this is the first time its happend Im some what confused with what's going on.