Why does under my tongue hurts?

So many factors..: This is such a broad question with so many possibilities. You want a definitive answer, not a guess... That's why you need to be examined locally by a dentist, oral surgeon, or your physician.

Trauma : I am going at this answer the dental route. It can be an simple as trauma caused by a toothbrush, which will go away in a week or two. If you have swelling and tooth pain as well it may be an abscessed tooth and you need to go have a dentist evaluate this. If it is something else a dentist will direct you to someone else, and main this is to go see someone.