I had a very similar sounding lesion. It started on the outside of the labia majora and slowly spread toward my leg and toward the inner part of the labia majora. It was shingles. The itching passed after a couple of weeks, but the pain was there for months afterward. Two years later I still have times where the area of origin is painful, though no lesions show up.
By the way, my father had psoriasis. It itched badly, but was only slightly painful when he broke the skin scratching. On the other had, the pain of shingles can be quite acute. Mine certainly was.
Mary