I've been feeling this weird fluttering sensation in the middle of my chest, and it's really bothering me. It seems to hit me especially hard after I eat or when I've been doing some work, and it makes it tough to breathe. I also get this faint feeling that I can't shake off. I'm not coughing exactly, though there are times when I feel like I might start. On top of that, my back has been aching, and it's hard to stand upright. What could be causing all of this?