If you're on the pill, you don't ovulate, so while on the 1-week pill break, you're not really having a period, but withdrawal bleeding. So if you're pregnant and on the pill, you might have this withdrawal bleeding that might look just like your period.
Some pregnant women experience bleeding during their pregnancy.
So even if you're not on the pill, bleeding alone does not always mean that you're not pregnant. So you would be able to tell at some point. Hope this helps. Somehow, my last question wasn't answered. This information is provided for educational purposes only.