What Causes Rapid Heartbeat After Exercise?

I am 68yrs male, weigh 210lbs having just lost 15lbs in 6 wks - no history of heart problems, always been fit, work out for an hour 5-6 times weekly for 20 yrs since stopping smoking. Vegetarian, cholesterol ok. Presented with chest pain on exercise. Had myocardial perfusion test which Cardio nurse writes, shows a small apical defect which does improve on the rest images and this could indicate an area of reversible ischaemia. Have not yet seen a cardiologist. Given beta blocker, statin, soluble aspirin and GTN spray (which is very effective - need it 6-8 times daily). Am I likely to have stable angina? Why, when I exercise now does my heart rate go to 150-160 when I am not putting much effort in, have no pain or breathlessness and it used to be 110-120? Should I stop exercising?