Why am i suddenly allergic to sep mushrooms?

Allergic Reactions: Time frame according to the U.S. National library of medicine, an allergic drug rash may not appear after the first exposure to a medication. It may take several doses or even a week before your immune system is triggered and you break out into a rash. Contact dermatitis often appears more quickly, according to the american academy of dermatology.

Maybe Not: Maybe you aren't allergic but simply had an adverse reaction due to the quantity of mushrooms or due to something else (like a new food, alcohol consumption, exercise, medications, etc). See an allergist for evaluation and testing.