Why do I get these sudden headaches?

Every so often, I’ll get a headache out of the blue. No warning, no build up, just BAM! headache. They’re usually not too severe, and a good nap takes care of it pretty well if it doesn’t go away on it’s own.

I did give up caffeine a little over a week ago, but haven’t had the serious side effects other than cravings. I’ve had the headaches before I quit, but they were less common.