Itchy anus can be due to undigested food like citrus fruits, grapes, certain spices, tomatoes, certain medicines like antibiotics and laxatives and certain drinks like
beer, milk, tea, or coffee. Anal fissures, hemorrhoids, threadworms and other worms can all cause anal itch. Itch can also be due to irritants like synthetic fiber, toiletries or fungal infection.
You should consult your doctor for this. The first step would be to get your stool examined for worms and undigested food.