decreased heart rate

For the past 4 months I have been extremely tired, had lightheadedness and shortness of breathe when walking. I can no longer take long walks or go up hills for fear of fainting. In June I had my blood pressure taken and heart rate. My blood pressure is fine - but my heart rate was 55 bpm. My heart rate has always been 80 bpm. I had a stress test (tread mill) and the doctor said my heart rate was going up and down - 109 then 60 etc. I then had a MIBI test and when on the tread mill, my heart rate would not increase. I was lightheaded and feeling dizzy but my heart rate would not increase. Now that all the tests are in, I've have been told that nothing is wrong with my heart.
Can you give me some advice, please.
JC