Today I woke up and I had one drooping eyelid! help!?

Yesterday my left eyelid slightly hurt but I ignored it and today I woke up and my left eye felt heavy so i looked in the mirror and i saw that it was slightly drooping! And I am not old so my muscles can't be over-exercised or whatever. I was not born like this. My parents have never had this so its not like anything genetically. What the heck do I do???