They did not start suddenly. I've been having them at fairly infrequent intervals for years - I think since I was a kid. At least, I have strong memories of a couple of nights when I woke up with bad headaches and nausea in which vomiting made it all better. And I've returned to these memories each time I had something like this happen. They're why I'd been certain it was a stomach thing.

It's that I had two such in about ten days as opposed to several weeks that I got concerned enough to speak to my doctor.