What Causes Palpitations Despite A Normal Holter Monitor Test?

I was prescribed azithromycin 500 mg. (1 tab daily for 5 days). I m unsure whether I want to take it for my sore throat after reading that it can cause heart arrhythmias, especially in older people. Here is why: I am 68 and have always had low blood pressure, but I started measuring it frequently after having some palpitations. When I take my blood pressure the machine detects an irregular heartbeat roughly 25% of the time. The cause of the irregularity is not known; I recently wore a Holter Monitor for 48 hours and it showed nothing irregular for that period. But ...only three months ago my adult daughter lived through an episode of Sudden Cardiac Arrest and was diagnosed with Polymorphic Ventricular Fibrillation. If something is going on with my own heart, will this medicine make it worse?