If bleeding tends to occur with oral contraceptives, it is caused by the side effects of the pill. Oral contraceptives such as short-acting contraceptives are likely to cause irregular vaginal bleeding due to the combination of estrogen and progesterone, which is not regular and will be relatively small in amount and mostly bright red in color. But menstrual blood is dark red and menstruation is regular, this is the difference between the two. In addition, if you take emergency contraceptive pills, most of them will have a small amount of vaginal bleeding 3-5 days after stopping the pills, which is far less than the amount of menstruation, and usually will be cleaned up after 3 days or so, so this is the main difference between bleeding from birth control pills and menstrual bleeding.