What Causes Itching In The Posterior Anal Region?

I've been treated for a yeast infection with diflucan twice and I'm using a Premarin vaginal cream for dryness due to Sjogrens Syndrome. But my anal area especially just above the anal opening to the back is still inflamed and itches and stings terribly. I don't have a problem with bleeding with bowel movements but it feels like my "butt crack" is tearing upwards. I've been using Tucks for relief of the itching but it really burns the posterior anus. What is this and what can I do to heal it?