It's kind of embarrassing to say to your doctor, "I have an itchy anus," but if Consider that you might have a fissure or small crack in the anal skin. An itchy and sore scalp is a common symptom associated with a variety of scalp diseases. Source:http://www.ehow.com/how-does_5499673_causes-itchy-anus.html
A sore on the inside of your butt crack can be the result of chafing (especially if you wear a thong), an ingrown hair, a herpes outbreak, a cyst, or other medical condition. You should have it evaluated by a physician.