My 11 month Male cat was attacked by 2 Dogs (boxer/rottweiler mix)!?

[solcyn1]

We were able to get the dogs off, and rushed him to the vet. The Vet examiend him in the waiting room and thought he had dislocated his hind legs. About an hour past and he gave him a pain injection (Torbugesic, ie. Morphine) and Pennecilin (for the nics from bites, there were no puncture wounds) at the same i asked the Vet to make sure because he doesn't seem to be in that much pain...turns out he was just sore.
But he already administered the meds!

Now for the REAL PROBLEM:
Under his (the cat) chin, it started to sag.
It doesn't feel like there is liquid or hardness.
buy he seems uncomfortable to lift his head.
I figured he was still in shock until the sag started to show 2 days later.

Could the Vet have overlooked this?
And will the Pennicilin last this long?

P.S.-The dogs had him pinned down and by the neck,
I say pinned down because they were not shaking him in the air .

Any advice would be great