The thing under my tongue is red, swollen and very swollen! what is wrong?

Salivary stone: Swelling under your tongue may be due to a distal salivary gland stone. Sialolithiasis (salivary gland stones) is the most common disease of the salivary glands. Almost half of the submandibular stones lie in the distal third of the duct (located under the anterior tongue) and are amenable to simple surgical release through an incision (cut) directly onto the stone. I suggest seeing an ent.

Trauma/infection?: You probably cut it or traumatized it some way from rough or sharp food. It may be infected. Try rinsing with warm salt water several times a day to see if it gets better on its own. If it continues to get worse or doesn't heal within 5 to 7 days see a dentist to get it checked.