Why does one of my eyebrows sag?

im 15. only 1 eyebrow sags down and makes 1 of my eyes look really small, sometimes It doesn't sag as much and the size difference of my eyes isn't as noticeable. I think it is worse when i don't get enough sleep but i'm not sure. The eyelid crease on the side it sags is much closer to my lash line than my other eye. what could cause this to happen? how can i fix it?