What Causes Soreness In The Armpit?

I am a 68 yr old female in good health. I had a mammogram in Dec. 2014 and all was fine. I have been experiencing tenderness in my left armpit area for about 3 weeks and after looking at the model of lymphatic system it would seem the discomfort is in Auxillary lympathic system. No lump just a spot that is sore, tender to the touch. What do you think and how should I proceed if this continues to bother me. I go to Nurse Practioner at M.D.A.C.C., Houston, TX. Thank you, Cindy T.