Does your employer have more than 50 employees?  Have you worked there more than 12 months?  If so, you are eligible under the Family Medical Leave Act (FMLA). Though most people think of this for long term leave, there is actully "Intermittant FMLA", which can be used for conditions that cause random disability, like Migraine.

My boss used to have a problem with me taking time for my migraines, and they are a lot more frequent.  Once I got FMLA approved though, it kind of made her understand.  It basically says "According to the Federal Government, I am disabled on these days".  I also made a point instead of saying "I need to take a sick day", I would say "I need to take an FMLA day".  Also, it will give you some job security as far as your absences are concerned.

Also, have you had a one on one with your boss?  Just because someone is an RN doesn't mean they know all that much about migraine.  I've been to neurologists who barely know what they are talking about.  I had to do that with my boss.  I told her that head pain is the symptom everyone knows about, but actually the least disabling symptom I get.  I gave her a run down, with a little "too much information" for shock value, and after that convo I think she really made a point to be more understanding about how sick I really am.

But first things first, FMLA. If your employer doesn't meet those conditions, speak to Human Resources about your options.

Good Luck!