What Causes Recurring Angina Attack In Person With Heart Disease?

I have heart disease, I have 3 stents and have had open heart surgery resulting in a triple bypass. Not the best way to spend an afternoon. So now I have angina and take Ranexa and it has helped a great deal. However I just had an angina attack a little while ago. Got tired of waiting for it to go away by itself so took a nitroglycerin pill and after a few minutes it went away. Now it has come back but not as long lasting or severe but has gone away. Didn't last long. Now can I continue on or should I be relaxing, reading or something? Thanks