What Causes Prolonged Periods?

I was on my period for about 13 days and the last few days there was a little bit of green on the tampon. I m not itchy or anything. Now there is a little bit of a foul odor. I m a very clean person and I ve only had one sex partner for 2 1/2 years now.What can cause that? And what do I need to do. I ve only been off my period for 1 day now