on the assumption that nothing is wrong with you medically, I wonder whether your diaphragm is literally “stuck” and cannot expand properly. It seems to happen when you are placing great demands on your cardiovascular system. That’s when proper breathing techniques will allow for complete expansion.
There are many techniques, and one of them is the Foundation Training my Dr. Eric Goodman. Here is a link to his 30 day challenge https://www.youtube.com/user/DoAFounder. You can check there for other exercises he suggests.