Kodak is feeling the shots all of a sudden

I don't know if I'm doing something different or if he's tender in the areas I've been giving him shots but tonight Kodak definitely felt his shot and was rather pissed off at me! I don't know what's happened. He's been fine up until a couple of days ago....I don't think he's lost any weight-and I'm making sure the needle is in the proper angle so as to go in as easily as possible-both times I gave it to him in his hind quarters-lets see if tomorrow when I give it in his fore quarters if it's better or the same. Of course all this is happening two days before I'm leaving town!