Sometimes I feel difficulty in breathing. I eat healthy food, doing exercise, and runing daily, and I do not feel any problems at that time. I do not feel any pain in my heart or lungs, but for two to three days, sometimes my nose is blocked due to a cold. I want to know if this is a serious problem and why this happens. I do not know if this is due to any stress or to exercise because I exercise and do cardio twice a day. So tell me, what should I have to check with the doctor?