I don't get them often but I think it's always stress for me. The first one was in comp school and I seriously wished to be dead with that so serious sufferers have my sympathies. My first sign is usually blind spots. Reading something and you are aware of a white spot just out of vision which isn't there. You cannot fight through it I have found and the only answer is straight to bed in a very dark room for a few hours. Trying to brave it out for me means loss of speech, vomiting and severe pain lasting 24 hours at least.