Cats can pant and it can be a sign of heatstroke. In the case of an adult cat it would be unable to stand, may cough, gasp for air, and the gums would be a dark red. As it is a kitten I would have her checked out by a vet, to rule out any heart problems.
My black cat used to do this whenever he was sitting in the summer sun for too long. He was very healthy and lived till nearly 20.
Don't want to put you off having your cat checked out tho,but it's probably over-heating for one reason or another!
Thanks it could be they are too warm they all eating well and active will keep an eye on them ..taking dog to vet for routine check so will ask ..twinny