Hi, My 4 Year-old Son, Was Hit On His Head

Hi, my 4 year-old son, was hit on his head on sunday evening...he didnt have any marks or swelling, during the night he got a high temperature and vomited.....we though it was because of something he ate (stomach infection). We treated the infection and he didn't vomit anymore but he's had fever everyday since. Most of the day he is fine, he eats and drinks well and has regular bowel movements, he gets the fevers only in the evenings for a short time and goes away without medicine. Could the fever still be from the blow to the head which we didnt think was too serious or might the stomach infection still be active? thank you