I started my period right before i turned 15, they were never completely regular...but i had them til i was close to 17. Since then i've had 3 or less periods per year, when i was 18 i didn't have any at all. When my bf and i started have sex i had my period regular for about 4 maybe 5 months ( i guess bc of an increase in hormones), but they left again. I'm about to turn 21 and still don't have periods. Im a long distance runner, which i know can affect this, about 5'8 127lb, which isn't unhealthy at all. I'm prone to getting stress fractures and i heard my lack of periods could be part of the reason for this. What should I do? even when i've had to stop running from injuries my periods didn't come back. Are there long term effects from this?