This is caused by a small blood vessel in the front of the eye leaking and a small volume of blood is spread thinly over the white of the eye underneath the clear layer at the front of the eye called the conjunctiva.
This always looks dramatic however it is painless and gets better on its own without treatment usually after a week or so. In most cases it happens spontaneously and is not significant however in some cases it can be caused by high blood pressure. Once it happens you should not be alarmed, it can reoccur once or twice again as the offending blood vessel does not always repair the first time. If it reoccurs more than this then specialist advice should be sought.