46 is not too low for a physically fit young person who is very active. Remember lance armstrong who won the tour de france bicycle rate five (or more) times in a row? He was in superb physical condition and his pulse rate was 35. The better your physical condition the slower your pulse rate. There is a formula for computing a target pulse rate. It varies with age and general physical condition.
Here is a link for target pulse by age:
remember this is only for information..only a doctor can evaluate your heart rate