Cat breathes heavy at home, but fine at the vets

Hello all. My cat is 6, has a grade 3 heart murmur (stable his whole life) and he has had several bladder blockages. He’s neutered and an only cat, and it’s just me and him in the home. He’s an inside cat. I’m in the UK and have AC so it’s not a heatwave issue.

He just had another blockage, and was hospitalised for 2 night and catheterised. He had zero adverse reactions to any drugs or sedation that my vets gave him, and was sent home. I’ve never known him to hide from me or be afraid of me, but he was for about 10 hours after we got home, and since has slowly settled back in. He’s been peeing fine (yay) but since being home, his breathing has been much more abdominal than normal. I took a video of this and showed my vet, and they said to bring him in.

The vet looked him over, checked his heart and lungs and temperature- but the weirdest part is that while at the vets, his breathing went back to normal and didn’t seem heavy at all. The vet found no problem with him. His lungs had no fluid, heart murmur was steady, and temperature fine. When sleeping his breaths per minute is about 24, so within normal range, but still heavier than usual. There’s vet advised that if it doesn’t improve within a few days we can do another ultrasound scan or XRAY (had those while staying with them), however they advised that as he was breathing normally with them, it could just be that at the vets he masks his stress and then when home he can finally process the trauma of hospitalisation and it’s just taking time.

There’s been nothing new introduced in my home - no new scents or plants or anything he could be allergic to. He doesn’t have an infection according to the vets (he was checked and scanned etc during his stay in hospital) and the weird part is the fact he breathes normal at the vets! He has been on gabapentin for 3 years to try and help his blockage tendencies, so I don’t think it’s an adverse reaction to pain meds.

He doesn’t seem distressed, he’s eating fine, wants cuddles and attention as usual. I truly have no idea what could be wrong. If anyone has experienced this before, please give me some tips in the comments!

TLDR: cat breathes heavy at home but fine when at the vets? Seeking answers.

P.s. I have pet insurance, which is so lucky because he is a medically expensive little man.