Question about minipill

I was put on progesterone only birth control 2 months ago. I switched from combo pills. Last month I had a normal "period" when I had 4 days left of the progesterone pills. For some reason I did not have any period at all this month. I called the Dr and was told it's possible not to have periods on the pill but I also heard only a small number of women don't have a period on it. For those of you that have taken these pills did you lose your periods and what signs should I look for if I was to be pregnant?