What Causes Tiredness And Stomach Pain After Suddenly Stopping Concerta?

My 17 year old son was taken off Concerta abruptly due to rapid heart beat. He has taken this for over six years. since stopping the Concerta he has bee extremely tires, and complains of stomach pain like he is going to be sick. We to ER yesterday due to pulse at 120 and stomach pain. EKG fine, chest xray fine, did get 2 bags of fluid which helped a little. Today, has not moved from the couch and still complaining of stomach. Any ideas?