J.B.
Which shot was it? My oldest son had one shot a few years ago that was particularly painful. I think it was his chicken pox booster (which was interesting because they insisted that he had received a prior dose and I know for a fact that he hadn't but I digress). The booster caused pain, bruising, redness and swelling from the injection site in the upper arm all the way to his shoulder and wrist. I think they called it cellulitis and he might have received and anti biotic to treat the infection but it was a few years ago so I don't remember exactly.
In any case, when I called the office they explained that they see this reaction frequently with that kind of shot. If you call the pediatrician, you can find out if this kind of reaction is common or not and what, if anything, should be done about it.