Do you have cheek biting issue ,? If it’s recent occurrence it could be some blood clot related to accidental biting. Or it could be some small growth or lump or lymph node that’s inflamed. difficult to comment without more detailed assessment and examination.
If you are wondering if it’s cancer it does not appear like that but if it’s been there for 3 weeks or more you should get it checked out.
Next Steps
A consultation with oral maxillofacial surgeon is warranted.