Could be “excess” sleep. Usually happens if I go back to sleep. Between a rock and a hard place. Kids wake up early and I get up with them in the week. Saturday is my “lie in” day to not be just getting 5 hours all week haha
Discussion
It's a thing apparently
"For some people prone to headaches, sleeping longer than usual on a weekend or vacation can cause head pain."
https://www.webmd.com/sleep-disorders/physical-side-effects-oversleeping#1-3