Why do I get a headache and my head turn red after bending over?

Abdominal Pressure. Bending at the waist increases intra-abdominal pressure causing venous return to the heart to decrease. This backs up the blood being pumped from the heart to the periphery which for your upper body is your head. Increasing venous pressure causes increased pressure in the head which leads to that headache. That's also why your face turns red. Same thing from a valsalva maneuver.