The most common cause of a red spot on the eye after a dive is a subconjunctival hemorrhage. This is a collection of blood over the sclera (white part) of the eye. It is usually caused by a mild mask squeeze and does not require any treatment. A more severe squeeze could result in other injuries to the eye, however, so it is a good idea to see your eye doctor just in case. It is absolutely essential to see your eye doctor if you have eye pain, double vision, blind spots in your field of vision, or decreased vision after a dive or if you have a history of eye surgery in the past.
For more information on vision issues, see the complete article by Frank K. Butler Jr., Captain, Medical Corps, United States Navy on High-Pressure Ophthalmology from Alert Diver.