Child Got Hit On Head. Started Crying And Vomiting. Bump Appeared. Should I Be Worried?

Question : Dear doctor, my two year old son bumped his head on Monday night. He was jumping in bed and fell back, bumped the back of his head on the bedhead rail which is made of metal. He did not lose his consciousness, but was silent for a few seconds, then started to cry. And, when he saw my upset face, he cried even harder, and then threw up. Personally I think his vomiting is caused by his crying. However, I still had taken him to a&e. He is fine since, played happily there. The doctor checked him, everything is fine, and released him. However, he cannot rule out concussion because of the vomiting. Now, it's nearly two days, he has been behaving himself, eating, sleeping, playing all normally. So, I just want a second opinion that whether he has sustained a concussion from that head bump? Is a one-off vomiting really indicates a concussion?
Ps, he had no lumps or swelling on his head.

And, he had another head bump just the day before this one. He was standing up too fast and bumped the top of his head on a wooden frame in a playgroup. And today morning, he bumped the back of his head again on floor (wooden floor with rug on), when he was throwing a fit. There are no warning signs for the other two head bumps. But, three head bumps in 72 hours, is it too much? It make me worry whether there would be any accumulative effects?

Looking forward to your opinion, thank you.