Why is underneath my tongue on left side swollen?

Hard to tell. There are a number of causes and some questions that need answering, as well as a visualization before a diagnosis can be made. It can be an infection, a cyst, a blocked duct, or even a tumor. These types of problems need the attention of a local doctor. Please call an oral surgeon.
Possible Infection. See your dentist for an evaluation to determine the exact cause of the swelling and correct this problem.