You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is 40 yr old patient was taking warfarin 4 gms for DVT prophylaxis. She was given  ceftriaxone 8 hrs i.v. for a dental infection. Later the lady experienced episode of  vaginal bleeding and darkening of urine. What is the cause? and explain in detail?