baby bumped her head

My baby is 4 and a half months old and is starting to sit up on her own, me and her dad and my MIL were watching her sit and she was so happy and smiling, she would sit for a few seconds then start to fall and her dad would catch her. Then she was sitting and her dad was holding her arms, and normally she falls to the side, but suddenly she completely stiffened her whole body and straightened her legs which made her go flying backwards even tho her arms were being held, there was nothing that could of been done in that split second. Shes never done that before. Anyway, she was sat on her padded changing mat, but its wooden floor underneath and she bumped the back of her head. Obviously she didnt fall far, but she did scream and then cry for about 10 seconds until we distracted her and she was happy. She has been fine since, its been about an hour. Is there any signs I should look out for or should I be worried? I cant tell if its red or not because she has a big birth mark in the back of her hair where she bumped..