Do your cats pant?

Luna is one year old. Since she was a kitten, when she plays hard, chasing her string toy all over and jumping after it, she starts panting. I mentioned it to the vet on 2 occasions and they inspected her nose and listened to her lungs and said there was nothing abnormal. She also used to snore but I haven't noticed that anymore. She never stops playing, I always put the toy away when she starts panting, if I didn't I think she would play until she drops! So, her energy level is good. I have never seen a cat pant before and it seems like she doesn't get much play time.

Wesley and Buttercup both pant with tongues stuck out after heavy play. We don't stop, I just give them a bit of a breather, and then we go again, like rounds in a boxing ring.

The time to worry about panting is if they do it after only mild activity or worse yet at rest and its not very hot in the room or reason for the cat to be mentally freaked out, then there is probably a heart issue.

Many cats I've met are simply far too lazy to get enough of a workout to actually get to the point of panting, and when they get tired, they just stop playing.

Osiris pants like crazy after a session with the laser, and a lot of my kitties as well as some fosters I've had, pant when stressed, in the car, etc.

After a hard round of playing, Tumbles will pant. At first, I was very concerned but now, I just watch him for signs of playing a bit too long and hard and try to stop before it gets to that point.

Domestic cats pant from extreme physical exertion or stress mainly. But in a "normal healthy cat" this is not an extremely common occurrence in my experience.

In your case since it is from play it is nothing to really worry about. Just make sure your cat has access to cold water and cool place to lay down like a room with A/C or a tile floor.

Since you took your cat to a veterinarian and I will assume it is recently you probably have nothing to worry about.