What causes pain and swelling in sublingual and submandibular area?

Hello doctor,

My sister, a 15-year-old female patient complains of intermittent pain on R sublingual area and neck (submandibular area) graded 6/10 at rest and aggravated when eating to 8/10. Past medical history: She developed fever recently and was diagnosed with UTI and was given cough remedy. Persistence of cough prompted us to another MD consult and was diagnosed with pneumonia of the lower lobe. She was on antibiotics for 10 days. She started complaining of the pain on March 25. MD thought of tonsilitis and was asked to gargle warm water with salt. Up until now, there is still pain on the area. Grade 1 tenderness on the R submandublar triangle area, no fever, mild cough still, feeling of lump on the R sublingual area, pain when tongue presses on the area. Patient complies with MD recommnedation of drinking a lot of water and gargling.