See your Dr and get a referral to a Cardiologist.
My HR was always in the very low 50's until after a surgery earlier this year. After that, my energy level never recovered. I'm in my mid 50's and also have never been an athlete but did quite a bit of hard work (living in the country) the past few years.
At my Dr's my HR went from low 50's to low 40's. They did an EKG and it would run between 38 and 42. Referred to a Cardio and after tests, wearable monitors, etc. ended up with a pacemaker.
It's been only a few weeks but I have more energy now and feel better than I have in a year or so.
Bradycardia-dual lead pacemaker implanted 12/18/2015
Large hiatal hernia repair-Nissen fundo wrap 7/27/2015
Prostate Cancer@55. G3+3=6 T1C
PSA 2/2014: 5.5 7/2014: 7.00 11/2014 (6 weeks after HDR): 4.80
2/2015: 4.23 5/2015: 3.67 8/2015: 4.01 11/2015: 1.46 (Finasteride)
HDR mono-therapy (4 rounds)
No symptoms, problems, etc. until my PSA went up from less than 4 the prior year