Lump on Dogs back between shoulder blades

I am seeking some advice for my Dog.  She is a 10 year old Golden Retriever mix.  I notice a very large lump between her shoulder blades (grape fruit size).  Took her to the vet and he put her on antibiotics.  The lump went down considerably as it is now quarter size. However,,,, all she does is sleep now, morning and night and I have to beg her to get up to go outside with me.  She seems normal other than wanting to lay down all the time..... could she have a problem with a disc in that area?   Thanks in advance.