Chest pain and gym workout

I am 46 years old male.  I am in teaching profession in a college.  I was going to gym in my early 30s and then discontinued as I was doing Ph.D. at that time and had to spend time for my Ph.D.  But after that whenever I found time I used to do some exercise with dumbbells. I am married.  For the past 15 years I was having chest pain over my left side, some times back side in the left chest.  I consulted Doctor, and took ECG twice and doctor ruled out of any cardiac problem.  Before 7 years again I went to another doctor with MD – cardiac specialist, he again took ECG test and ruled out the heart problem.  My family doctor said that it was only muscle pain.  I started going to gym for the past one month and started doing workouts directed by the gym in-charge. Before joining gym, I asked whether I could do gym exercises at this age.  He said, yes.  I am every day doing the exercise.  Nowadays, I started getting the chest pain and difficulty in breathing not while doing the workouts but after sometime. (it used to come even before joining gym – last year I had and tested with ECG; again no heart problem).  Then I got fear and consulted my family doctor, he suggested ECG and the test again came with negative result (no heart problem).  He opined that it was due to heartburn and anxiety,.  He gave some medicines to overcome heartburn and anxiety. Still I m confused whether doctor was correct? Should I discontinue the gym? What could be my actual problem? Can I continue gym workouts?  (I am using low weights only)