It could be the beta blocker if it has a tendency to place your heart into the range of 'bradycardia', low heart rate, and the medical establishment feels that the upper limit to bradycardia is 59 BPM. That seems high, to me and to real physicians who practice cardiology (some of them), but really it's your body that gets to call the shots on this. Perhaps monitor your heart rate several times each day for the next week and see if it manages to sustain a rate near/above 60 BPM. If it is consistently lower, especially below 50 BPM, it may indicate a review by your cardiologist. And that question would have to be placed before that person.