Have you been masturbating a lot recently? It may have just swollen up due to a lot of chafing and friction if you were going at it continually over a long period of time. If that's the case then it'll go down by itself in a couple of days if you just leave it alone, but you could try putting ice on it, or some cream to soothe it.
If not, do what everyone else says and see a doctor, as you may have caught something.