Grandma, I assume you mean by having heart disease is that you have trouble with either cholesterol buildup in your arteries
or valves
or both. When you exercise, your heart rate will increase to get more blood to your heart. Also, a low blood pressure isn't always good, believe me.
That said, carvedilol lowers blood pressure, sometimes to an extreme. Were you taking carvedilol when your heart rate went to 150?