Yours is a good question. First, I want to help you understand hormone production in a normal menstrual cycle. After that, we can talk about how the pill works to prevent pregnancy and why, in some women, it results in skipped periods. Finally, armed with this knowledge, we can get to your specific question.
In a normal menstrual cycle, the brain produces hormones that stimulate the ovaries to get an egg ready for ovulation and to produce estrogen. That estrogen production thickens the lining of the uterus, getting it ready for a pregnancy. After ovulation, the ovaries then produce progesterone, which stabilizes the uterine lining. When ovulation doesn’t result in a pregnancy, both hormone levels fall, which triggers the shedding of the uterine lining, causing a period.
Birth control pills contain estrogen and progesterone in every pill. Taken together, these hormones suppress brain stimulation of the ovaries so that you don’t ovulate. In addition, estrogen and progesterone produce a stable and very thin uterine lining. In the placebo week of the birth control pills, there are no hormones in the pills and this lining will shed, resulting in a period. Some women taking birth control pills get such a thin uterine lining that there’s nothing to shed, leading to skipped periods.
If you miss taking pills, you’re at risk of ovulating. The highest risk occurs if you forget to start a new pack of pills after taking all the placebo pills. If you forget to start a new pack, you’re at risk of your brain triggering ovulation, which can lead to an accidental pregnancy.
Finally, let’s talk about your question. Based on your description of the problem, I don’t think that you’re pregnant. But you should definitely take a pregnancy test. The morning-after pill, although effective, isn’t 100% effective, but closer to 90%. Once you’re sure that you aren’t pregnant, I would recommend you continue taking birth control pills as directed. It sounds like you’re one of the lucky gals who have the side of effect of no periods, or significantly reduced periods, when taking the pill.