What Causes Spotting And Heavy Bleeding In The Mid Cycle?

I am 24 year old female - had a c-section in November of 2012. A few weeks ago I began spotting on and off for about 10 days (began a week after my last period ended) 3 days ago I started what I thought to be my normal period - however, its extremely heavy and I wasn t due for my period for at least 2 weeks. Now I m finding that I m bleeding from my urethra as well. Pregnancy test was negative and internal exam was normal . I had the exam before the heavy bleeding started. What could this be?