Hard lump 1 in from my dog's spine what is it?

my dog had his rabies shot 4 weeks ago.the vet said their might be a lump but it should go away in a few days but they refuse to see him they say its nothing but my friends dog had something like it on her back her vet refused 2 c hers. we wee nvr able 2 find out wat it was.The center of the lump is hard but around it its like liquid and the lump you can move back and forth with your fingers. he acts like it doesnt hurt but he stares down my hand when i touch it.Has anyone else experienced this with their dogs if so would you please tell me what it is im so worried its a tumor its 1 in from his spine on his right side.Hes a jack russel terrier mixed with a chiuaua i think i spelled that right.