Sounds like a subconjunctival haemorrhage - one of the small vessels in the eye bursts and the blood spreads out between the sclera (white part) and the conjunctival membrane which attaches around the cornea so vision isn't effected. Common causes are coughing and vomiting ie increasing pressure in the chest and therefore in veins. It often occurs without any cause at all but, in cases of head trauma can indicate a fractured base of skull so a medical examination is essential in these cases. I'd see a doctor anyway.

You'll have some interesting colour changes in that eye over the next few days.

500Nitro is right about detached retinas and also discs between the cervical vertebrae can be damaged - I had a disc slip and press on the nerves to my left arm when my 470 doubled on a cape buffalo. And, I suppose you've read the story of a small (around 10 years od) boy who was killed when he was allowed to shoot a 454 handgun which hit him in the head during recoil.

mike