Sudden shortness of breath when not doing physical work. Cause?

Sometimes when I m breathing , I feel like the air is escaping my lungs and I lose my breath. It happens sporadically maybe once a month. I remember it happening in college one time that I started to panic because I couldn t catch my breath. I wasn t doing anything strenuous. Just sitting in class. The times it s happened I haven t been physically active so I m not sure what is causing it. I m 30 now and I just got that shortness of breath and I m laying in bed watching tv. I m a few lbs over weight but in college I was in shape so I don t think it s my weight that s causing the problem. Any ideas what can be causing this?