I know that can be concerning so I got her a vet appointment for this afternoon, please keep your fingers crossed that she will be okay. I know they said she had a heart murmur but then they said it seemed to disappear. I'd say with panting even though it was slight it could be the heart murmur is back, that's my guess. I'm pretty scared due to losing pets all too soon in the past from bad diseases that would come on suddenly. I always have gotten my animals vet checkups and as soon as I'd notice something out of the ordinary I'd get them right in. I always be sure to give the best care that I can.