I think that your sumptoms can be due to both unstabile angina pectoris and PVC. Since you already had a heart attack it is very possible that you may have unstabile angina pectoris. Another coronarography should reveal if there are narowed coronary blood vessels causing symptoms like yours.
A single PVC (premature ventricular contraction) is not a big problem but if more PVCs occur successively you can get ventricular tachicardia which can be a very serious condition. PVC is probably result of the heart attack you had a year ago.
Risk for heart attack is higher in people who already had a heart attack in the past.