I'm 20 years old and my periods are somewhat irregular. For example-last month I started 3 days late, this month 6 days early. I'm not on birth control and no significant life changes lately. I don't understand why this is happening.
My periods were always sort of erratic like that then after I had my first child I stopped having them altogether for over a year because my thyroid crapped out. You should have some bloodwork done at the very least.