Should I take the medicine? Issue?

I've been feeling breathless and lightheaded (only while at rest, not during physical activity) for a few weeks so I went to the hospital today. The doctor checked, did a few tests and all were normal (ECG Normal, Chest X-ray normal, BP normal, SpO2 normal) - the only thing high was my heart rate at the time (109-110). Usually my resting heart rate is around 54-56bpm as I exercise frequently. The doctor prescribed me Prolomet XL 12.5mg which is a beta blocker. My question is, cause I generally am an anxious person and my HR might've been high by just being at the doctors - Is it safe for me to take this medicine as my HR could go even lower and might cause problems/symptoms? Any suggestions on what might the cause of breathlessness be? Could it just be anxiety as I'm normal during walking/running? Could it be an gastric issue? Kindly advice. Thanks.