Pubic hair symptoms and conditions

i grew up with many differently bladder problems. and almost every time when i was younger i was given bactrim for a uti or kidney infection. well eventually i was put on this medication so much i became immune to it so they stopped giving it to me for like ten years to see if i could continue using it. since that ten years has passed every time i now take bactrim i have a very strange side effect that i have never in a million years experienced before. after bactrim is in my system for about 24hrs i develop a rash. i know what your thinking normal but no not really. this rash like "burn" only shows up in my pubic area on the right side toward the top of where my pubic hair grows. and it itches like crazy. but whenever i go to itch it then it burns so badly it brings tears to my eyes it hurts. this "mark" only shows up in that spot. they have finally come to the conclusion that i am allergic to sulfa drugs. but even with that being said why doesn't the rash spread any where else. it is kinda embarrassing that it only shows up in that spot. now i'm concerned the ob-gyn wants me to get the gardasil injections and i was reading the ingredients in the vaccine and it contains hydroxyphosphate sulfate which i am concerned may be a sulfa drug which means i might not be able to have that vaccine.