Thanks for the quick replies.
WantingMuscle, that link didn't work but I did a search and couldn't find anything that seemed related. Can you elaborate? I'll keep trying, anyway. Thanx very much.

Verbatim, I will definitely re-focus on my breathing. Maybe my breathing is seizing during a certain percentage of the movement where I don't notice it. The intensity I am using on squats now is certainly more than I have ever done, requiring more concentration - it's probably no coincidence that the headaches are happening now.

I appreciate it, guys.