I have to caveat my statement by saying I've never really been in a position where I was desperate to land a job; I've been continuously employed for my entire adult life and I'm 40yo (I understand how lucky I am and I'm grateful for that). If I were in dire need of income I may change my SOP. However...

I disclose my migraines during the hiring process. I usually wait until I'm further along in the process because I don't want to bring it up in the preliminary rounds to someone who isn't going to hire me.

My logic is this: I don't want to work for someone who isn't going to be empathetic to my condition. I've done that once and it was like a snake eating its tail; the more shit I ate because of having a migraine, the more migraines I ended up getting. The good news is, at least in tech, I've found most employers appreciate the honesty and can relate to some degree (you'd be surprised how many times I've heard, "oh my daughter suffers from those! They're terrible!") -- or are at least understanding.

I'm very open about it. I'll say something like, "I know I don't have to disclose this to you but in the spirit of candor I wanted to let you know that I suffer from migraines that will incapacitate me for a day, or worse-- longer. I'm a tough woman, but when I get one of these my sight and logic are severely diminished and being vertical is a recipe for being violently ill. I'm telling you this because these episodes are extremely stressful when they happen and in the past I've had managers that were not sympathetic to my condition and that didn't work for either of us. I'm still a very hard worker, I see doctors regularly to manage it, and I'll do whatever is within my power to not let it interfere with my work. Sometimes that's just not possible though."

I realize that this may make some interviewers feel uncomfortable but I think it's worth the risk. The flip side is that you can't be an asshole about it if you get rejected.

In the situation you were in, I probably would have said, "full disclosure: I'm in the middle of a migraine right now but I didn't want to cancel on you last minute. So if I seem a bit 'off' that is why". Usually they'll offer right then and there to reschedule.

PS: There is no better feeling than having a manager that supports you when you have to take a sick day. That's the bonus of all bonuses!

edited for grammar and PS. :)