I cannot diagnose reliably from one photograph, but the visible red sore on the underside/side of the tongue most resembles a small traumatic ulcer or aphthous-type mouth ulcer. Common triggers include biting/irritation from a sharp tooth, spicy foods, stress, or a viral illness.

Please have a dentist, oral-medicine clinician, or ENT clinician examine it if it:

Persistent mouth sores need examination because, although most are benign, a non-healing ulcer can occasionally indicate something more serious, as noted by the NHS mouth-ulcer guidance.

For now: avoid tobacco, alcohol, and spicy/acidic foods; avoid rubbing it with teeth; use warm salt-water rinses. Seek urgent care today if there is rapidly worsening tongue/floor-of-mouth swelling, fever, drooling, or trouble breathing/swallowing.

With right-sided tonsillitis plus a one-sided tongue/oral ulcer, this may still be uncomplicated viral or bacterial throat infection with a reactive ulcer. However, the combination of unilateral severe throat findings means you should be assessed in person today or within 24 hours, particularly to exclude a peritonsillar abscess (quinsy).

Go to urgent care/ER now if you have any of these:

Peritonsillar abscess commonly produces unilateral sore throat, fever, swallowing pain, trismus, voice change, and drooling. It needs prompt examination and can require antibiotics and drainage. The NHS guidance on quinsy advises immediate assessment for these features.

If you can swallow fluids and none of those warning signs are present, arrange a same-day/next-day appointment with a doctor or ENT. They may examine the tonsil and ulcer and decide whether a throat swab or testing for strep/mononucleosis is needed. Do not self-start antibiotics.