Flexing abs creates painful leg cramps

I'm a 24 y/o guy, 5' 8", weigh 155lbs, take no medications, do not smoke, and have no medical issues.

Starting last week, I returned to the gym and noticed during leg press exercises that bracing my abs cramped my legs up, specifically behind my knees. Not a tight stretch, but a cramp, a "charlie horse." It hurts more the harder I flex, and starts at a 50% effort flex.

I can lift heavy cramp-free, but if I flex my abs, even standing or at rest, both legs cramp severely, behind the knee, maybe around where the hamstring attaches there? Not sure if this is a nerve pinch thing or what.

I've been regularly stretching my hamstrings and hips for the past 4-5 months, and stopped this past month due to increased business. Cramps only started last week, though I haven't been flexing my abs for about a month.