I had subconjunctival hemorrhage myself. It was horrifying, but painless. I just woke up one morning and looked in the mirror, and AACK! The white of my left eye was blood red. I looked like something from a horror movie. Thoughts of ice picks in the eye flashed through my mind. But it felt perfectly normal. The only indication of harm was the appearance.
I was overdue for my annual eye exam, so I called my eye doctor immediately. When I got to her office, she first reassured me that it was not as bad as it looked. You know those little blood vessels that show up in “bloodshot” eyes? This is caused by inflammation of the vessels in the conjunctiva, the thin membrane that covers the whites of the eyes. Allergies, colds, scratches and other minor injuries make them swell enough that we can see them. They look bad enough when they're just inflamed and the makers of Visine sell a lot of eye drops to “get the red out”. When one of those blood vessels breaks, it creates a subconjunctival hemorrhage that makes the eye really, truly red.
My doctor wanted to know if I had any of the recognized risk factors:
1. Heavy lifting;
2. Intense coughing or sneezing;
4. Emotional stress;
5. High blood pressure;
7. Eye trauma; and
8. Aspirin, warfarin, or other blood-thinning drugs or supplements.
No, as far as I knew, that red eye happened out of the blue. My blood pressure in the doctor's office was fine, as usual. So she told me it was most likely a spike in my blood pressure, for unknown reasons. Not serious if it happens once. If it happens repeatedly, I'm supposed to ask my primary care physician to see what's going on with my blood pressure.
It took two weeks to clear up completely. I wore sunglasses a lot to cut down on the screaming and fainting when people looked at me. OK, I'm exaggerating; the worst side effect was a few dozen rounds of, “WHAT HAPPENED TO YOUR EYE?”
The references below show photos of this condition. They also recommend seeing an eye doctor if you have any other symptoms along with a red splotch in the eye.