“Immovable” lump?

Over the past month, I've noticed a lump sitting about a centimeter below my implant and on top of a rib. It is “immovable" with regard to the tissue it sits in and the skin that sits above it, but I was wondering if this is what immovable means when it comes to breast lumps, since the tissue itself is obviously not firmly affixed to my rib bone (like any normal human tissue!). My main question is, could some of you describe what your lumps felt like specifically (malignant or not) and clarify what “immovable" really means?

Because I had DCIS, I am not worried about a recurrence of that same cancer, but because I have a very strong family history and was diagnosed at 27, I am concerned about the possibility of a new cancer in some remaining breast tissue.

I will be going to my doctor to have this checked out in any case, but as I've got a long road ahead of me trying to keep fear in check, it would be helpful to hear specific descriptions of lumps and accompanying symptoms, whether malignant, fat necrosis, or scar tissue. Even though I will continue to get lumps and bumps checked, it's always nice to be armed with more knowledge and context. Thanks in advance!