Ugh! I'm so sorry!!
I've also had an allergic reaction to hair dye that I had used previously. It turns out that it was medication I was on for panic disorder. It also turned me photo-sensitive (can't be in the sun or my scalp gets hives all over it).
I am able to use the hair dye by taking a larger than normal dose of Benadryl. Have you taken anything different recently? Any kind of new medication?