What Causes Irregular Menstrual Cycles After A Child Birth?

I am a 28 years old male. my wife is 27. we had a baby 9 months earlier. he is very cute and healthy. I knew things about safe period in wife's mesntrual cycle, but i was not right to calculate the time because her first mestrual was 6 months delayed after delivery. and when I calculated the time after that, I was wrong probably because of irregularity in her cycle. we have no any problem that could cause worry but some:

1. I am a regular user of Nexito 10 for about 8 months.

2. Her first delivery was a CS.

2. And other family problems which can be settled if the former two are okay.