The only way to find out for sure what's causing it is a quick trip to the vets. They can rule out anal glands and conform any infection if there is one. Archie had an itchy episode last year and he licked and nibbled himself sore, which then got infected. We didn't really get to the bottom of the initial cause of the itch - some sort of allergy most likely - but he needed cream and anti-biotics to sort the sore patches.