What Causes 13 Days Period And Lower Abdomen Pain?

I am 16. I am 5'1 and weight 95 pounds. I am very healthy. Normally, I have a 5-7 day period. I started on Sunday of last week and it is still going. It has been 13 days. The first seven days were like a regular period but it was not as heavy as it used to be. Starting from the 8th day, the blood amount decreased. I'm still bleeding however. Today, I had a pain in my chest (right in the middle of where the rib cages meet) and in my lower abdomen.