What is the hard baseball sized lump on my dog's shoulder?

Get her to a vet immediately.

It could be cancer...

My friend had a dog, with the same thing: a lump on the shoulder. They said it was just a ball of muscle.

Two years later, the dog had to be put down, because the lump was a massive tumor.

So go to the vet, get your dog checked A.S.A.P.