Our vet makes sure to give our guys only one vaccination at a time. However, the boys are both very sensitive to the rabies vaccine (they're half brothers, so it wasn't surprising when the younger one had the same reaction when he was given it as his older brother). They both developed a lump - which wasn't LARGE, of course, but everything on a Chihuahua is large! - at the injection site. It went away no problem in both of them, but took a few weeks. Gizmo is actually due for his next rabies shot in a month or so, and now that he's four years old and having already had it once, I'm curious to see if he develops the same lump.