Kitty Has A New Heart Murmur😢

Today my 3 year old cat was diagnosed w/ a grade 3 heart murmur. This is new, they've never noticed a heart murmur before. He hates going to the vet & was panting & hyperventilating the whole way there. I asked if that stress could be the cause but it sounded like no... Could mean that he has heart disease for some reason. He goes for an ultrasound of his heart Wednesday to find out what's causing it. He's a very happy, active, goofy cat. I would have had no idea he had a heart murmur. This cat is my baby, he means so much to me. I've been crying and beside myself all day😢 Has anyone else gone through this? I need some help.