Of Course the cautious answer is to get checked by a Medical doctor.

You mention exercising for a "few minutes" 

I recently started exercising, after a long lay off.  I felt chest pains.  I started noting down the length of time I exercised.  I jhad started with 20 minutes, but cut back to 10 minutes, and started building back up.  The idea, from my experience, is to stay within your COMFOT ZONE.  If 3 minutes is your starting comfort zone, then do 3 minutes every other day, or 3 days a week.  Frequency of exercise, every other day, is my goal. I stop if I have a cold, or if I am short of sleep. 

You don't mention your heart rate.  I ride an electric bicycle with a heart rate monitor.  Most hotels have these.  I monitor my heart rate, keeping it moderate, until the last two minutes of my exercise, and then the last two minutes, I try to increase my heart rate by more vigorous exercie. I like the electric bicycle because I can regulate the effort that I am expending, by my rpm.  A real bicycle going up and down grades does not appeal to me.  I set the electric bicyle on Manual so I can push the arrow up or down, to regulate my difficulty resistance.

My theory is that most any heart problems can benefit from measured exercise.  Myocardia (small sized heart) is supposed to be contraindicated for exercise.  But trained atheletes hearts have become larger than ordinary hearts, by exercise training.  I have friends who are popping Doctor Prescribed pills and getting stints installed in their arteries or veins,  but not exercising. 

How are you regulating your urinating?  Void your blader before exercise, to increase your endu;rance.

I have recently been using EnduroxR4 from GNC and Running Stores, for replenishing my body systems after exercise. I don't know for sure if it works, but it FEELS real good.   A cooling down period is considered important by many.  You don't mention your plan or strategies for cooling down after your exercise.  My heart rate says over 100, as I am cooling down, and I count the cool down time, with a heart rate over 100 or 105, as exercise time.  Are you giving yourself full credit?

You don't mention walking during periods of your exercise or bike riding.  If I feel I am reaching the edge of my comfort Zone, I will slow from a jog to a wlk.  I will hop off my bicycle and walk for a while.  You do not discuss regulating the strenuousness of your exercise to keep an increased heart rate, but not beyond your comfort zone.  Your comfort zone will become wider, as you exercise more days.   

 It is common for people starting to exercise to over-do it at first.  Have faith that wherever you start, you will gradually increase your cordiaovascular capacity.