Exactly! You should have obtained a prescription for a follow up diagnostic mammogram from your FP. Was the breast center at the hospital the same place you got the first mammo? If not, they had no way of knowing you were there for a follow up for a specific problem. The prescription/referral from your FP would have indicated the area of concern/focus to the Radiologist. By presenting yourself, without a Rx/referral, the breast center probably submitted the claim as a screening mammogram.
IMHO it would be a real shame for you to refuse future mammograms, especially given the fact that you do have a previously documented breast lump. Honestly, this was YOUR mistake. No one else "messed up".

