If not allergies then technique is the next likely cause. Finally, there is the blade. A damaged edge may scrape the skin causing irritation, causing too much burning from an A/S. Also a blade that is not sharp enough may cause you to put too much pressure on the skin which will cause irritation causing burning as well. I know that if my blade is not up to snuff one of the signs is irritation on the cheeks.

Regards,

Ignatius