Itchiness around the anus is not the classical symptom of worms which people believe. I am not for one minute saying this can be ruled out, but firstly look at her physiology and diet. You say she is a long-haired cat. If she eats her own hair some of this will pass all the way through, and will inevitably be uncomfortable to pass.

When long hair is trapped ‘half-way out’ she may need assistance such as by rubbing herself on the carpet. However, I suggest you take your cat to the vet for de-worming medication and an examination just to be safe.