What Causes Shortness Of Breath And Abdominal Discomfort Upon Sitting Down?

When I sit in my lounge chair or computer chair and start to doze off I begin to have breathing problems and some discomfort in the upper abdominal area. It's a terrible empty feeling like I am going to die. I have even went to the ER for this a few times. They checked out everything and found nothing wrong. They said it is probably an anxiety attack. How can I have an anxiety attack when I am relaxed and dosing off. Sometimes I will get up and walk around the room for 20 or 30 minutes and that bad feeling goes away. I'm at my wits end about this and doctors really don't know what is causing this. Maybe you can help. Thankyou