Sheela-Na-Gig by Bridget Doyle

Jane Claremont, a not-so-innocent 19th-century English novice, is just about to be ordained as a nun when she begins to experience sensual visions -- and ones that have the knack of becoming true. This news reaches Lady Elaine, a benefactress of the convent, and she entices Jane away with the promise of a better life. While Jane is greatly attracted by Elaine's beauty, she starts to feel there is a more sinister reason for the other woman's interest