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You do not say how long it has been since the dental work.
If it has been a short period of time, then it could be due to inflammation and some swelling related to the dental work. In this setting, it would be reasonable to use a hydrogen peroxide mouth rinse and to take an oral anti-inflammatory medicine, such as ibuprofen or naproxen.
It is also possible that there could be an infection. However, an infection usually develops after a couple days or more. An infection would also be more of a concern if the symptoms have persisted. If there is an infection, then it would usually need an oral antibiotic, so it would require being seen by your dentist or by a doctor.
If uncertain, it would be fine to try the hydrogen peroxide and anti-inflammatory medicine initially, and then see your dentist if the symptoms persist despite these interventions.
If I can provide any clarification, please let me know.