I am in the military, and was doing sit ups, this past weekend. In the process of one of my sit ups, I had a sharp pain, almost like a spasm in my rib area. It took a while to catch my breath when this happened. I have since been home 2 days, and it is still continuing, and bruising of the skin on my lower (L) rib has occured. It is still hurting (a burning and stabbing pain) that is not affected with breathing, but is affected with moving and touching the area. What could have caused this?