I used to get exactly the same thing - faint, weak, sweaty, nausea, the runs. I'm in my 40s now, and things began to improve in my 30s - it does get better eventually!
Your GP might consider something to regulate hormone levels, but my guess is, it's normal. The way I dealt with it was to start taking painkillers as soon as you know the period is beginning - that helped with the runs as well, for some reason. I take Anadin Extra, which includes caffeine, a nice cup of coffee (caffeine helps with the faintness, for some reason) and keep some chocolate by you for something easy to raise your blood sugar quickly if you feel weak and nauseous. In short, be gentle with yourself for a day or two, and treat yourself!
I believe this sort of migraine is oestrogen-regulated, so you could ask your GP specifically about that, if it's a problem.
Good luck