Why has my one eyelid suddenly started drooping? (Photo)

Recently, just this past week actually, I noticed my one eyelid was drooping below the other. My face has always been asymmetrical so one of my eyelids has always been slightly below the other. However, this was unnoticeable unless you were to get really close. But now the drooping is much more obvious and noticable. I'm only 16 so it couldn't be from age. Also, I know about ptosis but could it be anyting else? And if it is ptosis how much would surgery cost and would insurance cover it?