About a month ago I was lifting weights up off the floor and afterwards I felt a cramped, burning sensation in my sternum. I felt like I couldn't breath properly. After about a week of this I went to the doctor who told me to have an X-ray. No broken ribs and lungs are clean. I can't excercise without feeling out of breath and I'm losing sleep because I can't get comfortable.

Does anyone know what my problem is? Is there anything I can do to make it better?