I was going to suggest that perhaps he hurt himself. Abby acted the same way at about 5 or 6 months old. I don't know what she did, but I can tell you she acted like a little monster when I picked her up. Is he a little less active than usual? Is he sleeping more and generally wants to be left alone? He probably just pulled a muscle - perhaps in his houlder area or something. Keep an eye on him to make sure he's eating and using the litterbox as normal.