Hi Katia, the swelling might be a collection of lymphatic fluid or blood, or just soft tissue swelling. The whole area is pretty sensitive. I had a major bleed under my expander at 3 weeks post mastectomy, and it spread to my under arm and I couldn't put it down either. It needed to be drained and cleaned out in theatre. The pain could be related to your swelling, which is pressing on nerves. I would definitely check with your doctor, but hopefully the pain will go once the swelling subsides.
Love Chris xx