Allergy :-(

They tell you to do the patch test before you dye every time. It's when you mix a small amount of the dye and developer together, put it on like the inside crease of your elbow, and leave it undisturbed to see if it gives you a reaction.
But doing that doesn't always help for me, as I seem to have a reaction to peroxide only sometimes, at random intervals. It's really weird, sometimes it'll make me swell and make any wounds I put it on not heal if I use it, and sometimes it's fine. Hair dye sometimes makes my head sore, and sometimes not. I dunno if you're like that or not, though. But if you are, you could patch test fine the day before dyeing and still get a sore or itchy head the day of.

have you done the ptch test yet to know if your are really allergic to it.