Itchy ring like rash above anus, goes onto both "cheeks" has been there for couple weeks. getting larger, not spreading to other areas. what is it?

Fungal or psoriasis: Fungal dermatitis or ringworm can cause intense pruritic ring shaped, expanding rash, with erythematous base. However, plaque psoriasis can also cause scaly, erythematous rash, which can also presents with pruritis. Treatment for fungal infection and psoriasis are vastly different. Consult your physician to determine and differentiate the cause of your rash is recommended.

Anal rash: Most likely you have a yeast infection of the anus and buttocks. It can be very itchy. You can use over the counter anti fungal cream such as miconazole or clotrimazole twice a day applied to clean dry skin to treat this rash. Treat until the rash resolves. If it does not resolve, seek medical attention.