A subconjunctival haemorrhage is caused by a bleed of a small blood vessel on the white of the eye.
The blood accumulates between the tissues on the white of the eye causing the eye to look very red. Generally the eye is not uncomfortable, and you will only notice something has happened if you look in the mirror.
The haemorrhage can occur due to a trauma to the eye, but also from heavy lifting, coughing, sneezing or vomiting. Occasionally it can be linked to high blood pressure or the use of blood thinners.
Generally a subconjunctival haemorrhage will resolve on its own after a week or two.