You are doing something called a ‘Valsalva” maneuver. Basically, by holding your breath, and unconsciously closing the valve at the top of your windpipe, you are increasing the pressure inside your chest. This also increases the pressure in the veins that drain blood from your head, so the blood backs up, making your face redder and more congested. The other answer about blood pressure going up and causing aneurysms is completely wrong. The effect on your arteries (where blood pressure is measured) is for your blood pressure to actually go down.
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