Could be an allergy but it might not be the pipe, that could be a coincidence. Could be heat burn, again, not necessarily from the pipe, could be coincidental happening from too-hot coffee, tea, soup, anything. Could be acid (but far more likely alkali) burn. We see it happen with tooth-whitening products. They use peroxide as the active bleaching agent, and for many people it causes pulpal sensitivity, but I've also seen it burn gingival tissue, albeit moreso in the early days of tooth-whitening where the patient was given trays which they filled with gel from a syringe. Nowadays with the premeasured delivery systems it happens much less often.
The way to check is to stop the pipe. If the condition doesn't clear in a week, it definitely isn't the pipe. If it clears, then it might be the pipe, so re-challenge. If it comes back quickly, it's the pipe. Well, most likely a certain tobacco. You can do the abstain/re-challenge thing with various tobaccos to isolate the culprit.