Welcome to the forum.
I also had pain in my breast before diagnosis, a dull aching pain that came and went. Thank goodness I did as when I mentioned it to my GP he found the lump. It was palpable and I could see it when I raised my arm. I cannot believe I had never noticed it before. It was quite high towards the top of my breast, not where I was self examining!
When I was going through treatment (chemo/ surgery/radiation), the pain disappeared?
I had the same cancer as you and it was six weeks before I started treatment, so three and a half weeks sounds okay. Hard not to worry about every ache and pain I know. Good luck, with yout treatment. <3