My Toddler Is 22months And I Just Noticed Today That

My toddler is 22months and I just noticed today that he has a soft lump on the back of his head the only reason I noticed because I he got a haircut and I don't remember it being there at the time of the haircut . I touch it and it doesn't hurt him its kinda reddish size of a dime . he slip and fell on the kitchen floor and bumped his Monday night I don't know if that gas to do with it .