Can Internal Injury Occur In A Child After A Fall?

My 22 month year old son fell yesterday off a roundabout ride. He fell backwards and smashed the back of his head to the concrete pavement. Inititally he screamed for around 10 minutes and then calmed down. Immediately after this he fell asleep for around 3 hours compared to his usual 1 hour of nap time, but this was his usual nap time. He has seemed fine since then, no vomiting, no loss of conciousness, normal speech etc. Should I be concerned about an internal injury, there are discernable marks or bumps on his head? Thank you for any advice