I feel for you!!!
Have you ever got yeast infections in the past?? The horrible thing about yeast infections is that they NEVER leave your system... once you've got it - you've got it!
I got EXACTLY what you're describing almost every month that I was on BCP... it was always the week right after AF and it was hell! My Dr first thought maybe it was the pads but when I stopped using them it was obvious it wasn't.
Try a baby rash cream to sooth - it's thick and lasts longer than anything else. (Penaten is a good one)
I wouldnt' take actual yeast infection meds b/c as far as my Dr can tell it's not quite an infection, just more of an imbalance after AF that causes the irritation.
I know how awful it is... I feel for you!!
Oh and as a PP said - if you're hairless I find it a bit easier to deal with BUT don't try any hair removal when you're irritated... that'll just make it worse!