The National Library of Medicine says that the most likely cause of occasional severe headaches is stress and tension. They note that diet can also contribute to headaches. Watch out for: cheeses, nuts, and foods that contain caffeine like coffee, colas, and chocolate.
"In some cases a severe headache may be a migraine. Symptoms suggestive of migraine include, one-sided headache, throbbing quality, sensitivity to light or sound, nausea or vomiting. Sometimes these are preceded by an aura which may include sensory or visual changes. Some headaches may be a combination of tension and migraine headaches."
-- National Library of Medicine
Medifocus - Their unique in-depth health information on 120 chronic or life-threatening medical issues such as cancer, heart ailments, migraines, and chronic childhood conditions gives you up-to-date, trustworthy summaries and reports. As medical professionals, they understand that being fully informed is your first defense, so they tell you all about the illness, the top doctors, and the best research.