What Could Cause A Hollow Sensation In My Gut, Lightheadedness, And Heartbeats Felt In My Stomach Only After Climbing Stairs?

Hi, I m 41 years old. I m not overweight. My lipid tests are perfect. My Bloodp Pressure is always 117 / 75. I brisk walk everyday with no issues at all. My only concern is, when I climb up the stairs, ((after I stop movement when I reach the top)) I feel my heart beats in my stomach and a weird hollowing sensation right in the middle of my guts, with a very mild lightheadedness. No chest pain, and no breathlessness. Keep in mind that have a gastritis for the last few months and don t know if that is what is causing this. But yet again, it s only when I climb stairs. No doctor could tell me the reason. This is making me anxious, please if anyone could give me a minor idea of what it is, I would be greatful.