Does it feel like constriction? Since your breathing is already very constricted during an attack? Most asthmatics do a great deal of heavy chest breathing, especially when triggered, so it can be related to that. If your doctor isn't finding anything related to your heart, maybe notice how you're breathing especially when you start to get wheezy and see if your chest or your diaphragm is doing most of the movement. Years ago, my doctor did a lot of coaching with me to use my belly not chest and that helped a great deal then.