What Causes Chest And Arm Pain?

Hi Doctor, I have had a six way bypass in 06 and a mild heart attack in 10. Lipoprotien A was the culprit. I have a great Cholesterol panel and the lipoprotien A is down. Now I have chest and arm pain on the left again. I took nitro and the pain let up. I had a work up with my cardiologist, radioactive xrays and tread mill. All numbers and pictures were good. He thinks pain could be muscular/skeletal. If this is so then why did nitro easy the pain?