If you have pain, blurring of vision, discharge from eye, double vision, flashing lights or floaters in vision then you should see an ophthalmlogist. If you just have a small bruise, you may be able to just watch it for a while. Be on the lookout for a possible inject bite or some sort of injury to the area that you don't remember. If it is bothersome or getting worse or any associated worrisome symptoms, please see an ophthalmologist. It probably is due to rubbing the eye hard or possible sneezing or coughing very hard.
MJK MD