Max is going on 2 and he has always had this heavy panting thing he does even when inside and not hot. He seems to control it in some way because he will stop and raise his ears when his curiousity hits. Just wondering if everyone's lab breathes kind of heavy even if they are not hot.
Two of mine tend to heavy pant only in either warm weather or just after exercise...but when it is getting near to the time when we need to reapply their flea meds, the panting becomes much more noticeable and without either of the above factors.
Also, in one of our rescues, Indie, he pants (and drools) when he is stressed. If you've treated for fleas and its not the temperature, then perhaps stress? Life changes can cause stress...i.e. if we are out a bit more than usual and Indie's schedule is a bit off, he pants more...or if there are folk in the house he doesn't know very well...he pants more...
If none of these are factors, then perhaps it is time to speak to a vet. Good wishes!