Hi, kids!

I'm in day 23 of P90x Lean. Before doing this, while overweight I was fairly fit, doing circuits classes a few times a week and running 3-6 miles per week.
I'm really enjoying (in a sick, sadistic sort of way) P90x, but I'm finding that I have a throbbing headache every time after Core Synergystics. I'm not holding my breath, I'm warming up and cooling down properly, staying hydrated, etc.
This doesn't happen on any of the other videos. But after nearly a month of doing Core once a week, it still happens. Any ideas??
Thanks!
Jen