Hello and welcome to HCM,
Lump or swelling seen under the tongue can be a cystic growth called Ranula
It is usually a painless cystic swelling seen inside the oral mucosa
and lies beneath the tongue.
Secondary infection of Ranula may lead to pain , swelling, discomfort in eating and swallowing.
Such swelling can be treated effectively hua n oral surgeon bu enucleation or marsupialization of the cysts.
The cystic lining and the sac is surgically removed to prevent its recurrence.
Other than Ranula, lumps below the tongue can be sublingual salivary gland
swellings, infected tooth draining on the lingual area , cyst in relation to tooth , benign or malignant growth.
Exact diagnosis can be made only after clinical examination so I would suggest you to consult an oral surgeon and get the necessary tests and evaluation done which helps in correct diagnosis and treatment.
I hope this helps,