The ECG is a very good study but it has many limitations. It is very reliable at detecting problems that interrupt or change the electrical energy paths through your heart. The heart has two major systems though - the pumbing and the electricity. Sometimes one systems affects the other but sometimes not, too. There are plenty of true heart attacks, for instance, that demonstrate no or very little change on the ECG. As well there are plenty of benign conditions that change the electricity (from "normal") and turn out to be nothing more than a congenital malformation in the way a person's heart works. Finally ECG's are a moment in time - they are very poor at predicting what might happen in the future. In short, just because you had a normal or abnormal ECG, it may or may not mean anything and it certainly doesn't tell you what might happen tomorrow. It needs to be assessed in context of other markers or symptoms.