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!
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.