First, thanks, again, to everyone for all your input on this subject. It really does help to get so many different ideas and experiences.
After three different migraines in February, we may have isolated a trigger in hotdogs. DD is upset, of course, since she loves hotdogs, but this wouldn't be a problem if she didn't eat them, right?
Anyway, no real hotdogs since the third migraine, and no more migraines. I found nitrate/nitrite free hotdogs at Trader Joes this week, and now I'm a hero! (she's 13, so that won't last, but I'll take it for now)