Breathing Problems

After my heart attack in January I had a double stent put in. Initially I had a lot of difficulty with my breathing and was sent to the respiratory clinic for tests, which did not produce anything useful. Over a two month period the breathing got better and I was able to resume something of a normal life. However, over the past 6 months my dizziness has increased enormously to the point where I feel convinced I am about to die. Coupled with that, breathing problems have resumed. It feels like my nose is constantly blocked and I cant get any oxygen in. When I try to sleep I am constantly waking very suddenly as if I have forgotten to breathe. Consequently, this makes me extremely tired and anxious. Every time I have been to hospital or seen a paramedic they say my oyxgen levels are fine. Any ideas please? Thanks.