have you identified her belly button and made sure that isn't what it is? when i found my dog's belly button i kinda freaked and went straight to the vet.
My older dog(s) have the same problems. Get it checked out. My male (who just passed away from Diabetes) had multiple fatty tumors. However, my female dachshund has two hard lumps - such as you are describing - that the vet is going to remove and biopsy when she gets her teeth cleaned next week. He said they are too hard to be fatty lumps and he's concerned they may be cancerous. She's also getting about 7 small skin lesions removed that are suspicious. All of these manifested in the past few months. Best to get it checked out because, if it is cancer, the sooner it is treated the better the outcome.