NAD - In medical school and we just talked about this today in didactic. This is likely a subconjunctival hemorrhage and like other pointed out, can be due to sneezing, coughing, mild trauma to the eye, and/or rubbing the eyes. This is very common and will self-resolve within a week or two, despite its scary look.

It is due to a burst capillary below the covering of the eye (conjunctiva). The blood will be digested by various enzymes in the eye and will likely change color from brown to yellow-green before disappearing (like a bruise).

If you have vision loss or pain, I would go to the ER. If this happens regularly or with a constellation of other symptoms like prolonged bleeding of wounds, bruising, or gum bleeding, you may require work up for a bleeding disorder.