Can't get rid of trouble breathing....?

I've had shortness of breath kind of like an asthma attack that won't go away for a week. It feels like I can never catch my breath, or take a deep breath, and very heavy chested. I went to my allergist who prescribed me a different asthma med and emergency inhaler and it provides NO relief. I ended up going to the ER a few days later when it got a little worse. They gave me a breathing treatment and steroids, as well as chest xrays that came up clean. That was a few days ago and I still have no relief and just have no idea what's going on. I'm only 23, health, in shape, not a smoker, not allergies, not overweight, or other medical conditions. What could be going on?!