One of the causes of a red eye is subconjunctival hemorrhage. This happens when a small blood vessel under the conjunctiva breaks and bleeds. A red patch of blood appears on the white of the eye, but causes no pain or change in vision. In some cases, the hemorrhage is caused by coughing, heaving lifting or vomiting. It may also develop following eye surgery or trauma.
While it may look frightening, a subconjunctival hemorrhage is essentially harmless. The blood becomes trapped underneath the clear conjunctival tissue, much like a bruise. No treatment is necessary as the blood naturally reabsorbs within several weeks