Bristol Palin has obviously had some work done!
She has likely received a chin implant. She looks puffy, which is either because of the recent implant, or could mean that the increase in the size of her chin is the result of an injectable filler. Her lower lip appears to have also been injected. It is hard to say for sure, but her eyebrows appear to have been lifted slightly, giving them a greater arch.
The chin looks overdone. It kind of makes her look like a cartoon character...or Snooki...
But overall, I'd say that its an improvement.