Well the vet said that it is 50 to 70 % chance that it is a duct that had ruptured or 30 % chance it's cancer . We decided to take x-rays to see, if it is cancer, that it didn't spread into her chest. Her x-rays showed nothing unusual in the chest but the lump did show up which means it has some density lowering the chances of it being a fluid filled duct. She is home but going in for surgery tomorrow morning to remove the lump so fingers and paws crossed.