I don't know of any way to tell whether what you experienced was a period, you may be able to tell if your next period falls 21-35 days from then. This article from the Mayo Clinic states that:   "After you stop taking the pill, you may have only a two-week delay before you ovulate again. Your period would follow about four to six weeks after you take the last pill. Once ovulation resumes, you can become pregnant. If this happens during your first cycle off the pill, you may not have a period at all. Although this scenario is possible, it isn't likely."   The article further says:   "When you stop taking the pill, it can take some time for your body to return to normal production of these hormones. Typically, your period should start again within three months after you stop taking the pill. Most women ovulate promptly and have a period within four to six weeks. But some women, especially those who took the pill to regulate their menstrual cycles, may not have a period for many months."   They are pretty good at judging the age of the baby with your first ultrasound. That can help pinpoint conception.