Is it my trapezius?

Two years ago I went on a rollercoaster frenzy and think I ended up with whiplash. Eventually the pain subsided. I didn't have any problems. Then, this past Sept. I was on vacation and woke up after a long restful night sleep with incredible pain in my left shoulder. It felt like a knife was jammed on the top side of my left shoulder. I was also having difficulty getting a deep breath. I just figured it was an asthma attack since we were in cigarette smoke filled Vegas and it was a consistent 120 degrees outside. When I returned home, the symptoms persisted. I went to my physician and he said it was likely a strain of my trapezius muscle. After a few days of rest (no exercise) the pain went away. Just the other night I woke up and the pain is back, as are the breathing difficulties. Is this really due to a muscle strain or is it something else? It hurts just sitting / breathing, but the pain is unbearable when I try to bend down or tie my shoes. My inhalers have not helped with the breathing difficulties either. Any ideas?