My normal heart rate is between 75 to 85. Today it was 52. What could cause it to drop so much? I am 58 not athletic

Vagal Tone vs Meds. Most likely in your case is something called increased vagal tone. The vegus nerve is our rest and digest nerve, and if it gets stimulated -- by a large meal, changing temperatures, and sometimes positions, rapidly, and often following urination or defecation -- your heart rate may lower. Certain medications like verapamil can also result in transiently lower heart rate.