What Causes Shoulder, Neck, Chest Area And Armpit Pain With Swelling?

I have some aching pain in my left shoulder and neck. Had a very stiff and painful neck a few days ago. I think I strained it from carrying a bookbag full of student work. Neck feels better but still some shoulder pain. On a scale from 1-10, I'd say it's a 2. I also have a dull pain under my left armpit, which has been there on and off for several weeks. Have had sharp pain in left chest area, but it comes and goes and have not had it recently. What really concerns me is that on Sunday I noticed what looks like a bit of swelling under my left arm pit. I will admit I've been under a lot of stress lately and hoping this is just stress related, but the swelling concerns me. I have a mammogram and gyn appointment in a few weeks. I am 46 year old female, 5'5", 175 pounds, generally healthy but I get lots of aches and pains. Have recently been changing diet to whole foods and walking 3 miles 3-4 days a week.