Without having seen a picture, it most likely sounds like an aphthous ulcer.
Try to get a disinfecting mouthwash, for example with chlorhexidine, at the pharmacy. Furthermore, a pain-relieving mouth gel, for example with lidocaine, is recommended.
Get well soon!
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Klinisch tätiger Oberarzt für HNO-Heilkunde. Promotion zum Thema Schmerztherapie nach HNO-Eingriffen. Jahrelange Tätigkeit in der Beratung für implantierbare Hörsysteme.