Thanks for asking the query,
Painful lump under the tongue can be either a gingival abscess
due to accumulation of bacterial plague
and calculus deep inside the gum pockets .
Or it can be a periodontal pocket arrising from the infected tooth .
Visit to the dentist
get a checkup done .
Take an x-ray of the invoved tooth, if the tooth is infected go for Root Canal treatment of tooth and drainage
of abscess .
Get complete mouth scaling and polishing , drainage of the abscess and root planing of the teeth.
Maintain a good oral hygiene .
Use antiseptic mouthwash gargles twice daily to prevent infection and promote healing .
Use saline water gargles 3-4 times daily to give a soothing effect.
Hope this helps out.