Worrying Shot Reaction

My photography skills leave much to be desired (I cannot seem to hold the camera still and unlike my last camera, this one doesn't have image stabilization).  Still, I think you'll get somewhat of an idea of what we've been horrified by after Sean's kindergarten shots.  The welt is almost six inches in diameter today and the middle is starting to reveal a horrible bruise.  He was so sore on Wednesday that he could barely walk and couldn't bend at all.  Thankfully, I convinced him to just skip the outdoor play part (they were scheduled to ride bikes) of his last day of preschool and go in towards the end to say good-bye to his friends.  One of the other moms, who is a nurse, took a look at the welt and seemed as concerned as I was.  However, when I called the doctor's office, they assured me that this is within the realm of normal for bad reactions to a shot.  Really?