What Causes Periods To Come Early?

I was married six months ago, and was sexually inactive for eight years prior, I had two abortions during my 20's and now I am forty five. My periods were regular bur short 21 days. Since I have been married my period was changing cycle on avarage by a day per month, last month it was a 27 days cycle. Suddenly this month I had my period or probably blood coming after 20 days noting that the blood started as one spot after carrying a heavy carper, blood came for a day one pad per day. I do not see any tissue, and the blood is fresh, the first spot however was brwonish. Any idea on what could this be.