Hard little b*****ds are squirrels, I shot one with my old ultra in .177 straight through the head between the eye and ear and when I walked over to pick it up it latched on to my hand with all its claws and had to pull its neck. That was one reason I rarely shoot .177. You only have to hold one to feel how hard and compact they really are.
Everyone has these shots now and again don't worry about it you did the right thing.