Baby bumped his head

My 5 month old has been doing lengths in his cot, started at one end went to the other turned and round and went back to the other end all sliding on his back. Should be asleep but he's been a bit difficult today so figured he was happy and relatively quiet. When he got to the other end he bumped his head on the end of the cot. He seems ok he didnt hit it hard I can't see any concussion signs didn't cry or anything. After checking on him and putting him back down he's worked his way to the other end and is trying to reach up to his mobile. Should I panic or just keep an eye on him?