My Dog has a tumor on her chest right around her left rib?

we took her to the vet and he told us we had to get her fixed and that that would probably make it go away. He told us it was a buildup of hormones and that once she was fully healed it would start to go down. Well it's been about 5 months now and if anything it's getting bigger. when she lays on her back you can see this large puffy raised spot about the size of a fifty cent piece around her left rib. She's an 8 pound chihuahua. and we're not sure of her age but it's starting to worry me and the vet keeps telling us it's a buildup of hormones and i think that's bs. what should i do?