This might just be a personal "quirk" of hers, with no real problem attached at all.
But to be on the safe side I would mention it to your vet. She might have some problem with the jaw muscles.
It doesn't quite sound like MMM (Masticatory Muscle Myositis) as that usually presents with other symptoms in addition, and is very painful if they try to open their mouths. She apparently doesn't show she's in pain when she does manage to yawn?
But nevertheless, speak to your vet