Could be you are drinking too much coke, rootbeer, gingerale type drinks lots of salt in them. water can be your best friend so says my urologist.
One possible answer is that it might be a stye or a clogged pore of some kind. If it hurts or itches, or if it seems to get in the way of your vision, I recommend you see a doctor. I had one of those a while back, and I was given some Benadryl. The swelling went down right away and it hasn't come back. But, like I said, seeing a doctor is the best way to make sure of something.
Maybe a bite from an insect,are you in a warm climate ., or an allergy have you used an eye cream or drops ?