Thanks for posting your query.
The medium sized mass developed under the cheek may be due to-
Deep dental caries resulting in formation of periapical abscess
I would suggest you to get a thorough clinical as well as x-ray examination by a dentist
The pus/fluid within the bump has to de drained/curetted.
Root canal treatment
has to be done if required.
Maintain oral hygiene well.
Use soft-bristled toothbrush to clean your teeth.
Hope this helps.