What causes an irregular mc??!!

For as long as I can remember, AF has been visiting on the 19th of the month, every month like clock work, and stays for 7 days. In September it was the 16th and only lasted 5 days and wasn't as heavy as usual. And this month it started this morning, and is a lot more painful than usual.
​Has anyone else had this problem? Is it normal for it to all of a sudden change when in the past it's been so regular and predictable???