There are several possible causes of pain under the tongue: 1. ulcers in the sublingual mucosa, which is a vulnerable area for mouth ulcers, and ulcers can cause redness, swelling, erosion and pain in the sublingual mucosa. 2. herpetic stomatitis or fungal stomatitis, where the body is infected by the herpes virus, which can lead to dense patches of blisters in the mucosa of the mouth, including the sublingual mucosa, which are very thin and can break after 24 hours, leaving Vesicles and congested mucous membranes cause pain. The fungal stomatitis is mainly infected by Candida albicans and occurs in people who have been taking antibiotics for a long time, who are extremely weak or who have been suffering from chronic diseases for a long time, fungal stomatitis can lead to white patches or white pseudomembranes in the mucous membrane under the tongue. 3, mucous gland cysts under the tongue, due to trauma or local infection leading to blockage or breakdown of the small glandular ducts in the sublingual mucosa, so that mucus storage or leakage to form Mucous cysts, which cannot disappear by themselves, will have local stinging pain.