What should I do if I get a migraine in an exam

I get really bad migraines sometimes, as well as the headache I initially get this growing blind spot that eventually covers my entire vision so if this happened in an exam it would be over for me I wouldn't be able to even read the questions for a while. Eventually the blindness goes and then I get the headache, light sensitivity and will 99% of the time throw up anything I consumed that day even if it was only water.

This is on my school medical record already, luckily I haven't had them for a good while but this also means my medication is out of date. Over the counter meds do nothing for me either. It's possible they've just stopped but it is a constant worry that I will get one in an exam.
What should I do in this situation? Should I tell an invigilator if it happens and would I be allowed any lenience somehow or could I retake or something?
Kind of worried that it will ruin my shot at getting good grades

How long do your migraines typically last? If they're relatively short (say, 20 minutes or so) then ask your school to arrange for "rest breaks" during the exam. This is one of the exam access arrangements which a school can organise relatively easily. Once the migraine kicks in you put your hand up and request a rest break. They make a note of the time, and you can leave the room (under supervision) until the symptoms subside. You then return, and they add-on however long you've been out to the exam end time.

Also, they can request "special consideration" from the exam board, who can add on up to 5% to your mark to cover medical flare-ups and other similar situations which are outside your control and which adversely impact your exam performance. Details here.