Sudden dip in PRbpm. Please suggest.

I'm a 24 year old Male. I'm a Student. I'm a fitness enthusiast and into endurance training (cycling). 4days ago suddenly I got near to fainting feeling. I immediately checked my PRbpm and Oxylevels. Oxylevel was 98 but PRbpm was 58. I took some rest. After an hour while sitting in my couch i again felt Near to Fainting feeling with confused state of mind and light headedness. I checked my readings which were Oxygen level 98 and PRbpm 52. I called my general physician he prescribed me Lemon juice on few intervals. I followed the same. Next day too i got the same situations twice. I visited my physician he said there's a possibility of heart ailment. He gave me one Cardiac tonic (homeopathy) i am taking the medicine. But my PRbpm is fluctuating drastically. I'm worried about what has suddenly happened to my heart as I'm into fitness from 3 years and never such situation has occurred. Please suggest.