What would cause a blueberry sized knot to pop up in the middle of tongue? popped up within an hour and is very very painful?

Hematoma: Sounds like trauma from biting your tongue and you have a blood blister or collection, otherwise known as a hematoma. I would try and put ice on it to control the swelling.

Painful tongue: What's really important in this case is you see a dentist asap for an evaluation. We can make some educated guesses as to what's causing this but can't diagnose nor treat your problem.

See your dentist: Some conditions may require further assessment, tests and your dentist may book a follow-up appointment or refer you to a specialist to determine a diagnosis. Take care.