Under my tongue behind the teeth on the string like things when you lift tongue up i got a blister thing on it that flops around what could it be?

Frenum: The frenulum of tongue or tongue web (also lingual frenulum or frenulum linguæ) is a small fold of mucous membrane extending from the floor of the mouth to the midline of the underside of the tongue. It can be accidentally cut or blistered, and may keep the tongue from fully extending outward.

Proper diagnosis: It could be a canker sore on your lingual frenum. Then again it could be lots of other things. You need to see a dentist to confirm diagnosis and treat accordingly if necessary. Unfortunately we can't diagnose these types of situations over the internet sight unseen.