I noticed sometimes when I do some type of activity that requires me to breathe heavily like run up the stairs or quick run to the bus I then immediately feel very weak like just as if I was just about to feel faint but don't. Obvs anxiety is telling me it's my heart. But I had tests done and all and I'm trying to find a logical reason. I usually don't eat much in the day and not drink a lot but today I ran up the stairs and it happened but I had a bit of dinner. Then a sandwich then beans on toast then a banana. I had quite a bit of water too. I'm not big I'm quite skinny and I never exercise. So could it just been coz I still didn't eat. As much? Maybe what I ate today is still not enough for me? Or could it be the lack of exercise in my life?