It is my understanding that Lisinopril is prescribed to lower blood pressure, and in so doing, reduce the workload on your heart. Some of your heart muscle is either dead or close to dead or you wouldn't have had the stent put in. The fast heart rate and chest pain could very well be related to heart disease, that is, your heart is having to work too hard and the muscle is starting to hurt because it's on its way to dying.
I never had high blood pressure before I had two stents put in, either. And it's a bit surprising to hear of someone getting a stent at the age of 45. You should definitely reconsider.