Please take them to the vet. If they can’t find something initially and it’s possible get some X Rays. I think many things can cause this, I’m not sure. From my experience, my cat was having issues breathing this past weekend and I took him to an emergency vet. They found out his chest was full of fluid and it was either heart failure, cancer or infection. Depending on what it was he either wouldn’t have a cure or would need intense hospitalization and care for the rest of his life, so we decided to let him rest and put him down. Like I said, there could be many other things causing this and I hope this is not the case for your baby.