Just like a bruise on the skin, a subconjunctival haemorrhage is caused when a tiny blood vessel bursts. 

They are often caused by coughing, straining or heavy lifting but sometimes they occur spontaneously. 

Normally, there is only a  small amount of blood that leaks, probably no more than a drop. However,  the blood is squeezed out between the conjunctiva and the sclera below, and as the conjunctiva is clear it looks a lot worse than it actually is. 

They generally self heal over a week or two without requiring any treatment. However, if your eye is uncomfortable then using comfort drops, such as Blink Intensive tears, and a cold compress may make the eye feel more comfortable. 

Recent research has shown a small increase in the risk of stroke in patients with SCH. If it is the first time you have had a SCH then it may be prudent to see your GP for routine blood pressure and cholesterol checks.

They often reoccur after a month or two where the blood vessel has been weakened but if they reoccur several times then it may be worth having a blood test to rule out any blood clotting disorders.

If the whole of the eye is red and there are no visible borders to the haemorrhage, even when you look to the right and left, then this should be assessed by a medical practitioner urgently.