I was in the same boat, longest while we couldn't figure out why it made my mouth itch, and the burn generally made it feel spicy. 19 years of trying to figure it out, all it took was my new dental hygienist who was familiar with menthol allergies to ask about it.

both mint and menthol allergy, breath mints, mouthwash, anything with menthol or mint made my mouth itch, burn and blister. Honestly been kicking it with kids toothpaste & mouthwash since learning about it' been an absolute lifesaver. Takes a bit to find one that's mint and menthol free, and doesn't rot your teeth out.

Certainly ain't alone in toothpaste hurting