Allergic to sunbeams? Doubtful. But something physiological is happening to you. About one person in four will recognize this photic sneeze reflex.
Sneezing is one of your body's reflex actions, which occur when your muscles respond automatically to a stimulus without conscious movement. Generally a sneeze happens when an irritant enters the nose. Another is the pupillary light reflex, wherein your pupils shrink when stimulated by a bright light. All reflexes require that a message get sent along complex neuronal pathways in the brain. It's conceivable that an anatomical mix-up could cause unintended results. Cross the sneeze reflex with the pupillary light reflex and you might get both responses to the singe stimulus of a bright light.
Scientists don't yet fully understand this phenomenon. But it's fairly certain that its alternative name is sure to produce an uncontrollable groan reflex: autosomal dominant compelling helio-opthalmic outburst, or ACHOO, syndrome. Seriously, it's in the literature.