I've noticed that whenever I do something active, like quickly walking up the stairs, I get out of breath really fast, and my vision kinda goes dark. It's like I have to sit or lie down and breathe deeply for a little while, maybe 5 to 10 minutes, and then things are okay again until I do something physical again. I'm kinda worried about this. What kind of doctor should I see about it, and what could be causing this?