This is one of the drawbacks to metoprolol. It does a fair job of controlling the high rate when in arrhythmia such as atrial fibrillation or supraventricular tachycardia. But, when the heart is behaving, it can beat down the rate to the point of bradycardia, sometimes to a dangerous rate. Happened to me and at least one other person I know on the www. It is commonly reported.

Unless you were experiencing some kind of tachyarrhythmia, I don't know why he would have placed you on metoprolol. And if it is indeed a tachyarrhythmia, it's not normal....period. Further, these things have a way of progressing as the heart ages or endures stress such as morphological changes like collagen deposition in the myocardium and fibrosis.

It sounds like you need to clarify some things with him, or find another cardiologist for a second opinion. Right now you're not in any great duress or urgency, especially if it self-corrects every time. However, it would be nice to have a plan for a night, say zero dark-thirty on a Saturday, when you awaken to find your heart racing and it goes on for an hour, two hours, three...