39 y/o female with inflamed, itchy, peeling skin around anus for weeks. Last night had a bloody bowel movement (fair amount of blood, toilet bowl bright red). STI tests all clear, not been sexually active at all for years. What could this be?

See MD. A large amount of bright red blood with bowel movement requires evaluation by your MD.
See Dr. Probably local irritation and/ or hemorrhoids. Need professional evaluation because of other possible causes. Meanwhile use skin lotion on bathroom paper to clean anal skin. Do this after any bowel movement. Do this whenever the area feels wet. Do this whenever the area feels itchy or irritated. Clean around + inside anus with the lotion on soft bathroom paper.