What Causes Sudden Breathing Difficulty, Chest And Shoulder Pain After Valve Replacement?

my 89 year old mother had aortic valve replacement surgerty on December 13, 2010, at the Mayo Clinic and was progressing okay until this morning when she began experiencing some difficulty breathing and left chest pain radiating to the sternum and left shoulder pain radiating to her neck. My father called the local emergency room in their home town and told him to bring her into the ER. She has been seen in the ER and admitted to the hospital today. Her EKG was normal and she is on oxygen and has an IV, in which she is receiving some type of pain medicine. She had an angiogram done in October 2010 and had a stent placed in her left coronary artery about six weeks before her open heart surgery.