I have a white spot under my tongue and it hurts. Is this a blister?

Might be. Could be a blister or a clogged salivary duct or an ulceration. Most common = ulceration. Rinse with warm salt water and keep an eye on it. If it worsens, see your dentist.
Possibly. There are a variety of causes of sores and discoloration of areas of the mouth. They range from aphthous ulcers to cancer. See you dentist or primary care doctor.
Possible. Possible. Pain? If present more than a week, it should be checked by a dentist for diagnosis.