It sounds like you could be experiencing a bi-product of breast feeding, like perhaps a clogged milk duct and the breast being larger could just be from having more milk in it or a sign of infection that often accompanies breast feeding.

Your doctor will be able to know more after the mammo and may suggest an ultrasound to rule out if it is indeed a cyst or if it is something that needs to be looked at further (like biopsy) because while a mammo shows mass, the US shows if it is fluid filled or solid.

I know when I found my lump it was after a miscarriage in my 2nd trimester.  I noticed it because I was telling my hubby that my breast were returning to normal (not hard from pregnancy) following my D&C.  That's when I felt the lump and it terrified me.  I thought or prayed it was a bi=product - a clogged milk duct or something, but my mammo showed the mass exactly as the pamplets do - it looked like a sun, with white lines extending from it.  They told me that I needed a biopsy ASAP.  I was only 31 with no history AT ALL of cancer in my family.  The lumpectomy came back +, but clear margins were not obtained, so I was told due to the aggressiveness of the cancer I must get a mastectomy.  I did and 3 yrs later IT STILL returned!  Luckily my plastic surgeon found it while performing reconstruction surgery.  Now that is on hold as I go through radiation & then another dose of chemo.  They beleive that this recurrence is part of my original cancer cause one little cell was left behind and I did not have radiation, only chemo - then of course my adjuvant therapy, Femara failed to decrease the amount of estrogen the body makes and did not help to slow or reverse the growth of breast cancers (my cancer was hormone receptor positive and I am considered post-menopausal because I had a total hysterectomy)

So that is my story - my lump when I found it, was circled in a black sharpie marker to make it easier to track till my mammo, and to me I "felt" as though it did move slightly or hurt, but in hindsight it could have just been because I obssessed over it and was constantly grabbing it, squeezing it, and trying to move it - so after all that it probably did hurt some, but most likely cause I caused it to, lol! 

I completely understand how you are feeling and I hope for your sake that everything turns out to be okay.  In the meantime try not to stress, though I know how hard it is - and remember if you ever need us we are here to help!

Good luck!

Angi