After severing a very stressful 8 year relationship. I had a headache for 4 months straight. I went to a couple of doctors and a physiotherapist/massage therapist - but I didn't want to take drugs, so I just persevered.
Then one day, it just disappeared. It's been 6 years since then and I can't recall having a headache since. I truly hope yours goes away a little faster. ;-)
I'm sure that you realized when heading into the interview ... that in your situation, you are as much interviewing them as they are interviewing you.
I've had headaches before, not for months, but enough to not like them. Mine is nothing too serious and it'll sort itself out eventually. I only have a brain the size of a pea so the headache doesn't have much to work with.
On the interview thing, yeah you're right. I know. Been around for a while.