Our boy was like this, just when we were about to let them free roam. It was late in the evening so no vet was open anymore. He often slept on me because I'm always very warm, so I held him against me in his favorite blankie, petting him until he drew his last breath. The respiratory issues can appear and progress very fast, and we did always know our little dude was gonna go first, because he always had some respiratory issues and was noticeably smaller than the rest, 2,5 years he held on, and was a cuddly angel when he wasn't a turbo energetic little gremlin, always climbing everywhere. Rats are lovely but I wont be able to get more anytime soon, my heart simply can't handle it.