If it has truly increased in size, go with your gut. Get a second opinion.
A lump growing bigger that fast sounds like a fluid filled cyst (usually benign). I would get a second opinion or insist they aspirate the cyst and test the fluid for any abnormality.
I had a fluid filled cyst for years that the docs watched (and I was okay waiting and watching) until last year it had some changes in the look of the fluid, so they aspirated it. It was benign and now the darn thing is gone and I no longer get breast pain. Yeah!