Birth control

So I've been on birth control for 3 months. Before taking birth control my periods were really heavy and really bad cramps. The first 2 months I was on the combination pill, which had helped my periods a lot, they were a lot lighter and I had hardly to no cramps. Unfortunately, I did have migraines which was a reason I switched over to the mini-pill. I never took any breaks, I just automatically switched over when it was time to start a new pack. Since the combination pill regulated my periods, before being on birth control they were very irregular, and now I was getting them every 28 days, but ever since switching to the mini-pill, I'm a week late on my period. Is the mini-pill less effective in preventing pregnancy or is it normal for me to miss it?