A subconjunctival hemorrhage (or subconjunctival haemorrhage) is bleeding underneath the conjunctiva. The conjunctiva contains many small, fragile blood vessels that are easily ruptured or broken. When this happens, blood leaks into the space between the conjunctiva and sclera.

Whereas a bruise typically appears black or blue underneath the skin, a subconjunctival hemorrhage initially appears bright red underneath the transparent conjunctiva. Later the hemorrhage may spread and become green or yellow, like a bruise. Usually this disappears within 2 weeks. blank">http://www.emedicine.com/oph/topic270.htm

Although its appearance may be alarming, a subconjunctival hemorrhage is generally a painless and harmless condition; however, it may be associated with high blood pressure, or _trauma to the eye.