Yes, it's quite common. My father also had this happen to him. He had symptoms, but thought that he just had an ulcer because the symptoms at first were not the classic signs of a heart attack
. He continued to work for two months before going to the doctor, when he found out what had really happened.
Your mother sounds as though she tries to be healthy. However, even people that eat right and exercise and don't smoke or drink excessively can develop heart problems. Genetics plays a part in the health of us all, as does simply growing older.
"Heart attack" is a scary thing to hear, but it doesn't always mean that a person is about to die or that their life will be completely changed. Talk to your mother's doctor, and to her as well. Make sure that she follows the diet, activity restrictions if any, medication schedule, and any other recommendations her doctor gives her.
Best of luck.