Why irregular period while taking mini pills and nursing?

I was on the mini pill and didn't get my period until my child was nine months old. It has something to do with hormone levels caused by nursing. Your period will come back when you stop or decrease frequency of nursing F.Y.I. I was on the mini-pill, or POP (progesterone only)pill and nursing but still got pregnant. When my son was 10 weeks old, we had intercourse one time and were pregnant again! Maybe it was because I was only on it for about a month..Our boys are now 12 months apart..just barely! Some people do not have a cycle the whole time they breastfeed.