Headache next day following light kickboxing sparring, few punches to head. no symptoms during training. is it from neck, head muscles?( weak neck)

Cautious optimism-: A few punches to the head is a loaded phrase and without more details on the characteristics of the headache, it's hard to give you straightforward answer. One approach-if the headache is getting better, then just monitor yourself; if it gets worse, see a healthcare professional sooner than later. It would be a good idea to refrain from getting punched in the head for the time being as well.

Posttraumatic: Headaches after even mild head injuries are common. Worrisome associated symptoms such as nausea and vomiting, drowsiness, confusion, visual problems, persistent and worsening headache make an evaluation by a physician mandatory.

Headache: Headache is more likely from trauma to the brain from the punches. You should reconsider whether your sport is worth repeated injuries that could cause long term trouble.