My 22 year old daughter just found blood in her stool. She said it was a lot. She is on the birth control pill and takes allergy medication - Zyrtec, I think. She had a tonsillectomy when she was 10 and has never had any other surgeries problems or med allergies, has never been pregnant or had any children or abortions (to my knowledge). It s a serious symptom - we are going to the doctor tomorrow. But what is the most common cause of this in young women?