The underneath of my tongue hurts?

Tongue pain. See your dentist to find out what's cause your pain. It could be as simple as a canker sore or irritation from a sharp tooth or filling.
Be more specific! Are you referring to the part of your tongue where you can see some blood vessels or to the floor of the mouth? Can you see anything unusual where it hurts? Does it hurt in a specific point or is it more so an area of your mouth? Is your pain sharp or dull, constant or comes and goes? Is there something in paeticular you do when it hurts? During the past 2 weeks have you eaten chips, fish, etc?