I Have Had A Heart Bypass Some 6 Years Ago,

I have had a heart bypass some 6 years ago, since last christmas i have been suffering tight chest-breathlessness-heavey feeling, I was sent to hospital and had a stent put in, since then i have had xrays-jelly scan on the uper chest and other tests, all proved negative, yet i still suffer from angina attacks, what is the cause and what can i do about it.