It does that every morning when I wake up (for a couple hours usually), and also sometimes at random times during the day for ususally arround an hour. It had been going on for about a year. It's really annoying because it hurts and it makes it hard to go back to sleep if I want to (sometimes it does it at night too, when I want to go to bed). It's sort of a sharp stabbing pain. occasionally it's behind my right breastbone, but usually my left one. Usuaally it goes away  when I don't breathe, and comes back when i breathe again.

I'm an 18 year old girl with a healthy weigh and no known health issues. What could be the poblem?