Well, generally speaking, there are many causes which can lead to a subconjunctival hemorrhage, which is similar to an ordinary bruise on the skin. Also we can say that it's like a bruise of the eye. And for the causes, it is hard to say. While an injury to the eye or a head injury can be the cause. Of course, you should know that sometimes they occur after a bout of coughing or vomiting. Besides, they are associated with high blood pressure. If you have a bleeding disorder, you may be more prone to having a subconjunctival hemorrhage, and even nose bleeds or easy bruising be the cause too. To treat them, you do not need medical cares. In my opinion, you can just put some cool teabags on your eyes, and then have a good rest. And they can disappear in a week.