Hi Linda. I'm a few weeks in front of you. Like you, I had a large mass 65mm. I had a bilateral mastectomy and clearance on the left side two and half weeks ago. I too was surprised how well I felt just one day after surgery.
I had three drains and did have some occasional pain associated with the drains rubbing against nerves. One instance was particularly painful, but was quickly relieved once the suction was released on the drain. I am also having some problem with cording under my left arm, but have been assured that this will get better with time. My right side is recovering quite well, but still unable to do anything too physical, as I am still draining fluid, which is now syringed out weekly by my surgeon.
My advice....be patient with yourself and give yourself time to heal.
I am seeing my Oncologist next week, so expect to start chemo shortly after, followed by radiotherapy.
Hoping all goes well with your surgery. Lynne :)