Why has my menstrual period started 13 days early. it has never been irregular.

It is: Likely due to irregular ovulation. They may normalize but as you get older, prior to menopause, your hormones can work irregularly. It could also be related to anatomical issues such as uterine fibroids. Consider getting seen. Your doc may want to do a bx. To exclude a problem on the inside of the uterus, but this is very uncommon at your age ( endometrial cancer). Good luck.