Unfortunately, this is concerning. Your blood pressure and heart rate should increase with activity. You should have your heart examined throughly. A drop in blood pressure like this could be from valvular disease, arrhythmias, dehydration, or vascular disease from diabetes. Additionally, it could be caused by cardiomyopathy which is damage or dysfunction of the heart muscle. I would continue taking your medication as prescribed. Drink plenty of fluids. If you get dizzy or lightheaded, then you need to rest. If you have chest pain, you should be evaluated at the emergency room.