T.,
Last year, my son had his dTap (leg) and boy, that thing swelled up and was red for a few days. Then, for another few days, his thigh remained swollen. I actually measured it, at least a whole inch bigger in circumference than the other.
What the pediatrician told me was that sometimes, when the muscle is tense during the injection, that this sort of swelling happens. Did the ped give you a fact sheet on the dTap, or did you look it up on the CDC website? There are things to watch for with any vaccine, however,my guess is that unless there were other profound symptoms, this is to do more with the tissue/muscle reacting to the injection than the serum itself.
For what it's worth, when he had his most recent dTap, he was a bit more relaxed and we had NO swelling or side effects/reactions. So, note it to the pediatrician and please don't worry too much. That welt really freaked me out at first; it did clear up in about two weeks. And you can give him motrin if he has that 'punched in the arm' feeling. They also suggested using a warm washcloth on the area and gently massaging the muscle, too.
Hang it there. It's scary when we don't expect it, huh?