You don't have to have a lot of caffeine to get a caffeine withdrawal headache. Variation in caffeine intake is a common cause of people's apparently random headaches.

I forget the name of it but I believe that a few decades back here in Australia there was a brand of over the counter pain killers that had caffeine in them. It perks you up and can reduce the severity of migraines.

You have a headache so you take one but when it wears off you get a caffeine withdrawal headache so you have another one... Repeat until your kidneys fail (or the government finally bans them).