Greetings. This could be often due to worms, not necessary that we have to be able to see them. We could know for sure only by a stool examination or swab from anus. Other possibility is that if u have piles or fissure or fistula, the moisture may attract certain commensal bacteria or fungus to cause infection. Keep the area dry . see a qualified dermatologist near you . warm regards

Only a cream will not work. You might need oral medicines (presumably anti fungal). So visit a dermatologist. It is difficult to treat only after hearing your symptoms. Examination is needed.