Lately, I have been feeling as though I cant breathe and I have been getting really light-headed(dizzieness) whenever I stand up after sitting for a long time. I have been loosing my breath very easily too, I think that it might be because of my heart, because my heart is always rapidly beating faster and faster when this happens. I tried to take deep breaths but it still felt though as if I couldn't breathe, and was still very light-headed too. What do u think this might be?