I get pain like this if I've been walking fast/jogging or if I haven't worn my bra during the day (like on a day off or if I'm sick lol). I figure it's just muscle soreness. It could be referred pain from a lot of different conditions, or direct pain from lots more different conditions.
Does it feel like a muscle ache? Is it a stabbing pain, cramping pain or crushing pain? When you say 'under your boob' do you mean behind your boob, like your throat is behind your mouth? If it persists or gets any worse you should just make a doctors' appointment :)
My mother had gall bladder pain in this area. She stopped eating a lot of rich food and it went away. It could be muscular, it could me a lot of things. Call and go see your GP doctor. If it's too much pain to take, go to ER.
not sure of what it could be but u say it under ur boobs!!! i would make a doc appt and go see ur family doc!!!
How old are you? It could be growth pain or you did something extraneous.