Sorry for the typo. It should (of course) be 200x15/45 = 66 :)
45 is not remarkably low for an active young man. With a blood pressure of 132/80 (which is normal but not on the low side) it sounds like your heart is able to maintain your blood pressure even at a heart rate of 45. If you play hockey I assume you are able to raise your heart rate when you exercise.
A slow heart rate is only a problem if:
A) Your heart rate does not increase properly with exercise, and/or
B) You have symptoms like dizziness or shortness of breath when your heart rate is slow.
You don't need to be in exceptionally good shape (like Lance Armstrong) to have a heart rate of 45. Heart rate is highly individual, if you should use your resting heart rate to determine your aerobic condition you need to see it together with your max heart rate. You can use this formula:
Max heart rate (bpm) x 15 / resting heart rate (bpm).
If the result is above 60, your condition is great.
Say, if you max heart rate is 200, you have (2000x15)/45 = 66. That would be great. If your max is 150, the result is 50, which is slightly above average for a young man.
The number is the estimated VO2Max, by the way. Have fun :)
I agree 45 is a low heart rate, but if you have no symptoms such as dizziness, shortness of breath or headaches (not an exhaustive list) and you doctor told you you are fine, then I'd say don't worry about it.
Given you have always been below 60 and I assume the 54 was long term (not just one reading), 45 may not be so bad. For me, I'd check every morning (or other rested time) and carefully measure. If the HR continues to drop I would go see my doctor again. Be sure you are getting an accurate measure...I usually count for a full minute, not just 10 seconds and multiply by 6.