I'm flat when I'm new. I'm fat when you use me. I release my gas when something sharp touches me. What am I?

“A tire”

While a tire is flat when new and can become fat (inflated) when used, it doesn't release gas when something sharp touches it in the same way a balloon does.

“A pancake”

A pancake is flat when made and can puff up when cooked, but it does not release gas when punctured; it is also not something that is typically inflated.

“A bag of chips”

A bag of chips can be flat when unopened and puffs up when air is trapped inside, but it doesn't specifically release gas when punctured and is not typically described as 'fat'.