I guess I didn't notice or considered the possibility that Killua had a headache, because Killua never had that before. I think its a little sudden to suddenly introduce this symtom against Rammot. At least have it happen in his fight against Shoot so that there is time for this concept to sink in. All I remember in that fight was Killua getting shaky legs and hands, nothing to indicate he felt any pain in this head.
In the end though, Im glad Killua is his cool self again.