Whenever my cat scratches me, it itches, turns red and swollen, and the skin becomes raised (similar to a mosquito bite). why is this? am i allergic?

Deermatographism: Dermatographic urticaria (also known as dermographism, dermatographism or "skin writing") is a skin disorder seen in 4–5% of the world's population and is one of the most common types of urticaria, in which the skin becomes raised and inflamed when stroked, scratched, rubbed, and sometimes even slapped. It is, at it's immunological root, a type of allergic reaction.

Allergic to cat: If this happens only when you are scratched by the cat and nothing else, then it is likely an allergic reaction not unlike testing you with skin prick test. Do you have nasal and eye symptoms when you are in contact with a cat. By the way, cat scratch is not totally innocuous and may lead to a condition known as Cat Scratch Disease.