Some allergies develop or worsen over time.
Things that they have had before & had no problems with can cause reactions all of a sudden. So it could be peanuts, or 160b or any other additives, OR it could be like Glamour said, just something a bit acidic burning her mouth...

Sorry , I know that is no help at all

Maybe take her to be tested if you're worried?
FWIW we had DS2 tested years ago (for another reason) & it showed a mild reaction to peanuts, he had had them before with no problems or symptoms but we avoid them now JIC.

Hopefully it's nothing too serious