I Normally Have Very Low Bp (116/65 Approx) And Pulse

i normally have very low bp (116/65 approx) and pulse (under 60). Occasionally I can feel my pulse racing and that leads to some light headedness and shortness of breath. I feel like i am going to drop. It seems that this usually happens after an airline flight, but I am feeling this way today and I haven't been on an airplane! Is there concern for this? I have had a 48 hour heart monitor on and did a treadmill test (forget the name!) nothing shows up except an enlarged heart. I am 58 and otherwise in good health (former physical education teacher). I feel like I am in tune with my body and I don't particularly like this feeling!