What Causes Sharp Stabbing Pain Under The Left Arm?

Hi under my left arm I have sharp, stabbing pain that comes and goes. Feels tender when I touch it. I am wearing a cotton soft bra, I thought it was underwire digging into my skin. (that s what it feels like). I have had the pain for about a week now, when I lift or stretch my left arm it relieves the pain, when I slouch I feel it.