Sometimes I feel like there is a weight on my chest and this makes me feel like I can't get enough air in my lungs. My heart also feels like it is beating harder (not faster) than usual. This sensation lasts about 3 days, and is not related to my physical activity levels at the time.I do not have any allergies that I am aware of. I've never had asthma, or any other kind of respiratory issue.What could this possible be?