Baby fell from 3-4 feets height.

My 10 months old baby fell from 4 feets height. Seeking for suggestions. My baby boy is 10 months old. Today, he fell off my hand on a hard floor (tiled) when I lost balance due to watery floor. I do not remember exactly which parts of the body got directly hit, but he was lying flat facing the roof top. He must have fallen from almost 3-4 feets of height and to the best of my guess, must have hit the back of his head. He cried aloud for nearly 20 plus minutes and was in a shock of the intensity of the hit. I gave him a head bath and after that went on to feed him and he slowly fell asleep. There are no wounds externally at all. Today he slept for 3 hours and 15 minutes (an hour more than regular timing). After waking up, he behaves as normal. We did not notice anything unusual so far. we are a bit worried if he would have got any internal issues.  Can somebody advice what are the symptoms which we should seriously worry about and go to a hospital? any advice is most welcome. thanks