Accidental poke in the eye with a penis or accidental ejaculation in the eyes.
During sex you ask your man if he will cum in your mouth. When he's about to cum and pulls it out, you don't get your mouth on it in time and he ends up ejaculating the first part of his load in your eye, which in turn becomes bloodshot and sore for a while. It's easier to say you have "dickeye" from that point on until the redness and soreness goes away. Don't complain! You asked for it!