What causes blood in eye and how common is it?

Where?: If you mean on the white part and there is little or no pain, yes, that is very common. It is no more serious than a bruise to your thigh and will go away without any overt treatment. It is more common in those who need blood thinners--but do not stop them. The most common cause is stifling a sneeze--let her rip. Don't "play" with it. Don't hold your breath with exertion of any kind.