Because your skin is sensitive to the chemicals in it. You might what to look for something designed for sensitive skin.
Why does my face feel irritated after using an acne treatment cream?
You may be using toooooo much of the cream. Read your instructions again. If possible use less daily or every other day.
Probably because you are having an adverse reaction to it. Skin allergy or even just sensitive skin perhaps. Try another brand for sensitive skin.