Thread: 2 week period

I am 22 years old, and have been on the pill for 2 years. About 4 months ago I started bleeding a week early and my period lasted 2 weeks. The next month I was in the clear. But then for the last 2 months it's happened that my period has started a week early, and has lasted till the end of my actual period, totaling 2 whole weeks. I did run a marathon last year, and run at least 60 km a week. What can be causing this abnormal bleeding?