The funny thing about allergies is that you can not have a reaction one day, then have one the next to the same thing. I seem to be the queen of drug allergies. The contrast used for an MRI, the one that 1 in 1 billion people are allergic to, yes, I am. Typically, it is the epi that causes the reactions. There is nonepi options. I would tell the next dentist about what happened, and I would change dentist. Not that this guy is neccessarily bad, but that you have lost your trust in him.