Both of my babies got their rabies vaccinations at the same time. One of them developed the knot you are talking about, the other one- nothing. It didn't seem to be painful, so I just watched it for a couple of days and sure enough, it went away. I had already seen posts concerning this, so I wasn't too worried. (although my inital reaction was "OMG!!! I gotta call the vet!!!"
Then I took a few minutes to regain my senses...) Just keep an eye on it, you should begin to see some improvement soon. It was probably about 6 days before the knot was completely gone on my pup.