What Causes Pain In Armpits?

I have a rash under my left arm pit and every now and again I have painful sensations in the armpit area sometimes companies by th robbing sensations around my sternal in the center of my chest . I fell in the bathtub a week ago. . Pain wasn t really present before that. When I fell I grabbed hold of the side of the shower with my left arm. I have had 2 previous surgeries on my shoulder prior to this incident. Sometimes my back will throb as well? Any advice? Thanks.