it could be asthma. i was diagnosed with very mild asthma a few years ago, after i experienced the same symptoms as you - i'd just be sitting around or trying to sleep and i wouldn't be able to catch my breath properly. my parents kept complaining because i was taking really deep breaths to try and get comfortable and it sounded like i was constantly sighing. i think that kind of mild asthma can be cured over time, and helped by exercise, but that's not true in of all cases - i had been diagnosed as asthmatic as a baby, but my mum didn't want me to take medication for it because she thought it would go away on its own. unfortunately, it didn't.