I definitely feel your pain. I lost a foster about a month ago to a previously undiagnosed heart issue. He started acting weird on the way to his adoption, and I decided to put it off.
I called the e-vet, and they said to wait it out, since it sounded like it wasn't an emergency. The next morning he started having tiny seizures, and I went anyway and he was euthanized because they said he was on his way out. Apparently he'd had/was having a heart attack (by their best guess) and that's what was causing all the deep abdominal breathing.
Heart problems in pigs are tricky to pinpoint because it's hard to find vets with experience for them and it's difficult to pinpoint them since they don't always show up on x-rays.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry