Does she only do it when you hold her?
It can be because of something stuck in the nose that she is trying to remove, or puffing air can be a sign of being annoyed. Perhaps she is saying she doesnt want to be held.
However if you are concerned about any health issue, then you should see a vet as we cannot say definitively what is causing it in your piggy's case
Dominance Behaviours In Guinea Pigs
You say she is chubby? Do you weigh her weekly as part of routine care and have you checked her heft?
Weight - Monitoring and Management
Does she live with another guinea pig?