Apart from their hormones being seriously out of whack
, there is often a period of adjustment when cats return from the vet. They smell and sometimes act a bit different than before (at least temporarily), which can be seen as threatening. Don't worry, they'll come around. I've heard that putting a drop of vanilla extract under their chins helps, I suppose because it gives them a common odour. But time should take care of this temporary contretemps.