I had similar at nursery in the fact that they didn't notice or inform me. My boy didn't speak a lot, kept to himself and when hurt, would hide. So I pick him up from nursery, looked dark eyed, just thought tired. Gets out the car when get home, Rubbed his head, like you do, he winced and said ouch. I lifted up his hair and had a golf ball size lump with an indent in the middle. Finally get it out him, he slipped and head butted the corner edge of the wooden sleeper, sand pit. Gets him inside, was very sleepy, so called non emergency 111 or whatever it was, as my boy never runs out of energy, later diagnosed with ADHD at 7, so really not like him, suggested A&E, which I did, purely from own experience of cracking my skull as a kid, still have a bump 40 odd years later. Ring school and left a message, spoke to manager next day. He was fine, but he definitely wasn't himself for a while and kept him off a couple of days.
I would complain though, if he told them and they just dismissed him and got on with it. Normally they only take notice of the dramatic crying when hurt, most kids do, but mine is like urs, just sucks it up, would also log the kids name in your head, who did it and see if her name comes up again. Kids even at that age, can be nasty. But defo log this one.