What causes a period to last more than 10 days?

A longer, heavier period can indicate a luteal phase defect and/or lack of progesterone. You might talk with your DR concerning a progesterone regiment. This can make your cycles easier and shorter.

Also, a heavier/longer period can indicate an early missed miscarriage. If in doubt, check with your Dr who can give you an early ultrasound and blood test to see what is going on.

It can also be a uterine fibroid.