A 35 year old woman presents to you in clinic with a complaint of vaginal bleeding after sex. This has only started happening recently; she says, "I've had dozens of partners in the past and this is new." The bleeding happens during and right after coitus. She does not have menorrhagia, irregular menstrual cycles, or intermenstrual bleeding in the absence of coitus. There is no pain, dyspareunia, or dysuria. Her partner is asymptomatic. She has not seen any doctors ever, and is worried she has an STD. Exam does not show any lesions; the vagina and cervix look normal, without discharge. HIV, hepatitis, gonococcus, chlamydia are negative.
Challenge: What is the most serious potential diagnosis?