I feel for you dear. I went through something similar. I have bilateral mastectomies after I completed chemo. I never did radiation. About 4 days after the mastectomies the left side decided to pop open for no good reason. I immediately called the surgeon and was told to do wet to dry dressings until it healed up and then I would be able to do the expander / implant route.
For those that don't know - wet to dry dressings involve getting sterile water, sterile gloves, and sterile 4x4 sponges. I soaked the sponges and then shoved them into my open wound. Would then cover the wound with several layers of 4x4s. I changed the dressing twice a day and never, ever let tap water get on the dressing. That involved covering it with plastic wrap and tape to just take a shower. At first I used about a dozen 4x4s to stuff my chest but over time I could get less and less sponges into the wound. Hate to tell you this but it took 1 and 1/2 years for the damn thing to heal.
Now, if my situation had been like what you described I would have gotten a second opinion after 3 weeks of no progress in healing and still having pain. It shouldn't be causing you pain. I had no pain, even with stuffing the wound full of sterile sponges. Did the PS suture the wound closed? If he did you might have bacteria inside the wound. When a mastectomy site opens up (no matter the cause) you always run the risk of infection and the wound should heal from inside out to prevent this. That is what the wet to dry dressing do - allow healing from inside to outside.
Are you stuffing the wound with sterile sponges and water? Have you been checked for infection? (white cell count, fevers, redness, oozing????). An infection would cause pain for sure.
I know all this because 1 - I have been through something similar and 2 - I spent 30 years as an ICU RN. Gal, get a second opinion from another PS doctor or have your doctor refer you to someone who specializes in wound healing. Ask doctor to check you for infection.
It does take a lot of time for this kind of event to heal BUT it shouldn't be painful, except when you change the dressing. The pain is what worries me.
Please come back and let us know the progress you are experiencing, or if you have other questions and comments. I'm sure my friends here can also offer some help and encouragement to you.
take GOoD care