A subconjunctival hemorrhage is bleeding underneath the conjunctiva. The conjunctiva contains many small, fragile blood vessels that are easily ruptured or broken. When this happens, blood leaks into the space between the conjunctiva and sclera.
Such a hemorrhage may be caused by a sudden or severe sneeze or cough, or due to hypertension or as a side effect of blood thinners. It may also be caused by heavy lifting, vomiting, or even rubbing one's eyes too roughly. In other cases, it may result from straining due to constipation. Also, it can result as a minor post-operative complication in eye surgeries such as LASIK.