What Causes Dizziness Along With Discomfort In Left Upper Back?

I am 73 and have been experiencing a sense of wooziness/dizziness - rarely severe - though one time - the first time it was quite strong and I thought I would faint...had to grab the counter. Also a little breathlessness and a some discomfort in by left upper back. My heart sometimes flutters considerably. Is this a cause for concern. I live at 7000 ft, use a CPAP at night and have very mild mitral valave regurgitation. Last year, I wore a heart monitor for the weekend and they found nothing.