2 days ago I started punching things & my middle finger knuckle is bigger than usual & the side is swollen. And it hurts to use my hand. Is it broken?

Possibly. The only way to know for sure is to have it x-rayed. Have it checked out as soon as possible, because if it is broken, you want to make sure it heals correctly. If it heals incorrectly, it could affect the use of your hand in the future.