Hello, YellowBird,
It's exactly what happened to my son almost two month ago. He was hit by the soccer ball on the top of the head. Didn't think he had concussion and continued normal activities for a couple of days.

He told us about it only after headache was never gone for days and medications didn't work. He rested after for 4 weeks. Didn't change anything. he still has headache. Not severe, but annoying.

Everyone here, and doctors as well, suggest vitamins. Our doctor stressed on 400 mg of B2 a day. I hope you feel better soon. It might take longer that we hope, but we should be patient.