You could check her heart rate when she is doing this. Particularly while she has been in a quiet mode for atleast 15 minutes or so. Anything above 150 per minute or so should be checked out by your vet. High normal resting rate is 120- 130 but I think that puppies run a little higher than this.
I do belive its their was of cooling down. Panting is normal and different for every dog. Excessive panting esp after no activity should be monitored and brought up to your vet if you think its unreasonable.
From what I can tell it is mostly when it gets warm in the house...though it doesn't affect Bo the same...and it takes her a lot longer to stop panting after playing than it does for him..she also seems to pant more if she is thirsty than he does...
Panting is how they disipate heat. It is like us sweating. All dogs are a bit different like people. Some people sweat more or more easily, some dogs pant more. One major contributing factor in ability to disipate heat is the length of a dogs muzzle. Boxers are naturally not as good at shedding heat as say a lab with a longer muzzle. Now that being said, in your sig pics it looks as though bo's muzzle is longer than Jetta's. This may enable him to cool faster than her.