Is it possible to pass a fetus when you sit on the toilet to urinate when you didn't know you were pregnant and not start bleeding until the next day?

Yes, it is possible to be pregnant and have a miscarriage before learning you were pregnant. When you sat on the toilet to urinate, your body was in a position to easily pass the fetus, which may be why you passed it when you did. But if that is what happened to you, you need to see your doctor to find out why you didn't start your period until the next day, and to make sure the uterus shed the lining (afterbirth) as it should. When a miscarriage occurs, bleeding begins either just before passing the fetus, or immediately after.