The first question would be the duration of the lump, whether it has been observed in the last few days or is present since a long time...... based on the description and picture that you have provided, this could be a traumatic lesion which might heal with time...... but most likely it is melanotic naevus, which will never heal on its own and does not require any treatment as such if it does not show any changes of colour, shape, size or contour..... If any changes are observed immediate excision of the lesion is highly recommended.....
Do you suffer from cheek biting regularly? Please visit a dentist for proper diagnosis. It looks like a hemetoma ( blood filled boil)due to injury. Removal of wisdome teeth will solve the problem if you have those.
Based on the picture you have sent, it could be a Macule or Nevus - excessive melanin pigmentation.. like a 'Mole' as commonly said and seen if it is on skin.
Inside oral cavity, it isn't common.
Or it can be Amalgam Tattoo, if she has any Silver fillings adjacent to it.
Not of immediate concern but at the same time dont neglect, it may not go automatically. Go to a dentist, he may remove it with LASER or biopsy if he feels necessary.
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