Neck pain

If it feels muscular it probably is.  Consider that your body is now unbalanced with one breast gone.  A prosthesis is unlikely to exactly match the weight of the mission breast.  I was a DDD before unilateral mast in 2008. I had the left breast removed.  My left neck and back gets very sore when I'm wearing the prosthesis.  My right neck and back gets very sore when I'm not wearing the boob-in-a-box. (I am sooo looking forward to recon and bieng in balanced and not in pain by the end of every day).  The neck and back muscles work very hard to maintain balance, and that balance is delicate.  it takes very little to upset that balance. 

The swollen glands under your chin are unlikely to be bc related.  The lymph drainage from the breast area simply does not go in that direction.  Those nodes collect drainage from the head and face--making a sinus infection or brewing throat infection a far more likely culprit.  Of course, when my PCP told me that I had a sinus infection and enlarged nodes in my neck last year I promptly burst into tears.  It took her an hour to convince me the beast hadn't come back.  

Breast cancer very, very rarely spreads to the muscles.  BC likes to spread to the bones, brain and liver.  A met to the neck bones can cause neck and back pain, but it rarely feels muscular.  I've almost always heard it described as sharp or burning, or "like the bone is breaking."  Can you point to one spot that hurts or is the pain more general?  If you can't put one finger on the exact spot of the pain, it's probably not bone met pain.  

When you say you have pain at the mastectomy edges, do you mean along the scar or around the area where the breast tissue was removed?  Pain at the incision line could be anything from irritatation from the prosthesis or bra to nerve regeneration.  Pain around the edges of where the breast tissue was removed is probably nerve regeneration or irritation from the bra/prosthesis or clothing.  Because you also have pulling pain in your arm I wonder if you have the same thing I have--pectoralis muscle damage/irritation. I got the damage from radiation, but you can also get it from a mastectomy, especially if you didn't get physical therapy right after surgery and you are unconciously underusing that arm and shoulder.  This can be managed with some stretches and exercises that a physical therapist can teach you.  I notice the arm pulling and a more neck/back pain if i don't keep that muscle stretched out.  The more I use that arm the less pulling and back pain I get.  

All that being said, the best thing to do is call the onc's office and ask about the pain.  They know your particular situation and can advise you and reassure you, or get you in sooner if advisable.