Your Mother has all the symptoms that will make her have vital exhaustion. Her symptoms are a textbook case for coronary artery blockages. She needs to see her cardiologist very soon. Yes this could be a sign of an impending mi.
My Mother had 6X bypass surgery at 73 and did very well. I do hope that your Mother does not have to have bypass surgery.
Jack
Any change in breathing patterns with exertion needs to be addressed asap. I know nothing except my own symptoms, but being SOB (short of breath) has been a heart attack and on a recent occasion, a need for new stents and procedures to clean up previously-inserted stents. I would not delay in getting Mom to a Cardiologist.