When I was in college, I broke my finger hitting my cat. Now, lest you think I am cruel AND over-forceful… here’s the story.
I had made an extra sandwich at dinner because I had to pull an all-nighter. It was on my bed and I was sitting on the floor with my back up against the bed. Out of the corner of my eye I saw kitty on the bed, backing away with the meat out of the sandwich. I swatted in his direction and must have gotten just the right (wrong?!) motion, like cracking a whip but the whip was my arm. I connected with NOTHING, not the cat, not the bed, NOTHING, just air, and felt something snap. A nice spiral fracture of my finger, from… air.
My sister was concussed by her horse’s head (he was swinging for a fly) but I’m sure that’s more common than these dog injuries – I’m surprised at the number of stories! Hope the OP’s head feels better soon.