MY 83 Yr Old Mother Has Stage 2 Heart Dz.

MY 83 yr old mother has stage 2 heart dz. She does has occas. VPCS induced with over exertion. NOt severe enuf to warrant treatment per her cardiologist. TOday she came down stairs sat down and said she had chest pain. SHe put her head down and seemed on the verge of unconsciousness. He color was pale, cold hands but head was sweating. PUrple lips. IT lasted about 10 min. Then she seemed ok. I called 911 and they checked her out. he said ekg normal and normal BP. WHat do u think as potential causes of this?