What Causes Dark Coloration During Menstruation?

Hi. I just had my period a week ago. It was a light flow and lasted for 7 days versus my typical 3 - 5 day cycle. A few days later - I started bleeding again. At first, it was red and only after I wiped. Yesterday, it was globby and darker. I am not on the pill and I have not had sex for a few months. I did, however, just start a new diet and have lost 4 - 5 lb. over the past week. Any ideas? Thanks.