What Causes Pain Under The Armpit?

i have a sharp pain under my right arm pit to the right of my right breast. its so bad, that when i lay down, i cant get up, but if i squeeze the breast i did finally manage to get up. i have had some back problems down the right hand side of my back over the last few days, but put that down to carrying my 1 year old and the cough i have had for 1 month