jasper had a funny turn this afternoon?

hi jasper has hcm his vet said a year ago he only has about 6 months to live well he visited a couple of weeks ago and his vet was very pleased with him he is on fortekor which is keeping him stable, today he came in from his usual walk round the garden, he was a little bit sick with grass, then his breathing was really hard and fast, he sounded like a stream train, he was streching his neck out obviously to open his airways, this lasted for about 5 minutes then he started breathing normally, again, at first i thought he had gone in heart failure, this is the first time this has happened, is this normal for a cat with a bad heart, his heart murmur was first noticed 4 years ago,