Suspicous golf ball size lump under skin on dogs right hip area?

[beaglelover]

Hello Dr's.
I am glad to have found your website. I am greatly concerned about my 6 yr old Beagle Sparky, he had his teeth cleaned for the first time 3.5 weeks ago and the site on his hind quarter where the vet would have administered antibotic shot? is now enlarged the size of a golf ball. It does not appear to bother the dog. I took Sparky to the vet, and the vet said he was not sure what it was,
but that it could be reaction to the shot they would have given before surgery. Sparky is on antibiotics for 7 days, and if it does not go away the vet said to bring him back.
Concerns: it appeared very quickly, almost over night, and the fact that it did not appear until almost two weeks after teeth cleaning concerns me. Sparky has been on meds for 4 days now and the lump has not reduced in size. What's your opinion? The lump is movable, does not seem fluid filled because it is hard. Help....