70% or more of all the breast lumps are luckily benign, but can make for some major stress and worries. I'm the last one to tell anyone to go have your lump evaluated, since I didn't do it myself when I found mine over a year ago now. Stupid, I know, but my erradicle fears got the best of me. I wish you all the best though.
Hi, you should have the lump evaluated by your doctor, please consult him at the earliest. What you feel in the armpit could be lymph nodes, but its difficult to tell exactly what it is without examining. The armpit lump/tenderness may not be related to the muscle pull. Consult your doctor he will examine you and may ask for imaging studies and if needed may do a biopsy to study the lesion. Take care and good luck.