Ive Been Getting A Stabbing Flutter Like Pain About An

Ive been getting a stabbing flutter like pain about an inch to 2 inches under my left arm pit since End of Dec. I have had it about 4 times. It takes my breath away and hunches me over when it happens. What could this be? It has happened twice while on a hike and twice while working. I have been feeling exhausted and occasionally get a light headed feeling but i also have sinusitis chronically so have been using the neti pot but my daughter the OT is telling me it could be my heart that i am overlooking and not just sinusitis. What is your hypothesis?