See a vet. Only a biopsy/removal will be able to determine if it's a fatty lump or something else. Biopsy is a simple procedure where your vet will take a few cells out of the lump by doing a needle aspirate, putting the sample on a slide, staining them and looking at them under microscope. Its quick easy and your dog wont feel a thing. In my experience fatty lumps aren't hard, they are soft and jiggly and you can move them around under the skin. If it is hard like a golf ball it May not be a fatty lump, if it grows rapidly, is nodular and irregular in shape and wont move around much, it is probably not a lipoma(fatty lump).Dogs breed and age have a lot to do with lumps. These are all guidelines its not always this way but they give a fair indication. Chewing and scratching could indicate allergies. Once again, an issue for your vet. I'd get the lump looked at soon if it's on her chest. Hope it's nothing serious!
