Chest pain

My mom is 72 years old. Had mild heart attack at 69. Has COPD. For about the last 6 months or so her pulse rate has been high (about 120)but no one could tell her why. Her breathing recently got much worse, a week ago she thought she was having a major heart attack. Severe pain in her chest, she was admitted and treated as though she had one. But they say no, don't know what happened (5 days later)Potassium was a little low, placed her on 2 blood pressure meds and lipator.Right now breathing is good (for her)so is pulse and pressure. Could her heart been stressing from the rapid pulse? (she quit smoking almost 2 years ago.)