2-3 day period help?!

Lee

So I'm always very regular. I get a period every 28 days exactly and they last 5 days each time.

This month though it came 3 days early and only lasted 2-3 days. As well as being alot more painful than usual. Wasn't so much heavier but really painful. Anyone have ideas on why this could be? I'm not on birth control so that cant b affecting it.