Should not be of any concern. If you're maintaining a normal blood pressure with that rate its not indicative of any problems. If you do experience fatiugue, dizziness, lightheadedness, etc with a rate of 53-55 you should consult your physician. But if you feel fine, tis northing to be concerned about. If anything a low resting HR, especially when supine, is quite normal especially if you are aerobically fit.
Now if you were laying flat and totally calm, yet your HR was 130+, that would be problematic.
I agree, and note the beta blocker (BP pill) will lower heart rate. But the below 60 rate is great to my knowledge. Congratulations on the weight loss. I'm now a senior and afib prevents me from running or even jogging. But, in my middle age to 65 years I went from walking to 3 miles in 30 minutes (6 mph average) and think it helped me control weight and made any physical activity easier. Keep it up!