I have been on the progesterone only pill (Cerazette) for about 4 months, and ever since I started taking it I haven't had a period.

This didn't worry me as I knew it was a possible side effect, and my periods were very light anyway. I also took a pregnancy test and it was negative.

This last week I have had a period which surprised me as I had gotten used to not having any while on the pill. I'm a little worried that this means the pill has stopped working for me, or that it means I may need a higher dose.

Is this likely?

I know that some people continue to have periods while on the pill, I am just concerned as my reaction seems to have changed for no reason.