I have a Brittany spaniel male dog that is 7 yrs. old.
He is a hunter and I have noticed lately that after running for about 20 or so minutes his breathing sounds rattley or rasping. I ve also noticed a little bright colored blood coming out of his left nostril but I couldnt tell if it was a cut from being in a briar patch.
Something may be stuck in the nose or there may be a mass in the nose. Both would account for the heavy breathing. I do recommend a visit to the vet. Good luck, vetcare