What Causes Swollen Glands Under Armpit With Pain In Neck?

I have a swollen gland under my left armpit its been there about 2 months but I havent been sick, I am concerned because I also noticed a tender spot for a long time now above my left breast I thought it might be a gland also I had a mammogram last year No know problem stated my breast are dense, And for sometime I have had strange short sharp pain in my neck , I have nightsweats