Suggest Treatment For Movable Lump On Rib Cage

I am36 yr old female-- i found a lump/mass about marble size on my rib cage it doesnt hurt and i think it s a little moveable, i have a history of Non-hodgkins lymphoma last chemo was July 2013, i havent had a pet scan recently since I was pregnant and just had baby January 14 ,, I did tell my Oncologist but they werent able to palpate on examination, can it just be fat deposit?? I am very concerned since I have the NH history,,I am going to ask my Oncologist to take another look, and maybe have a Scan soon. Thank you