have you been on the mini pill for several months at least? and have been taking it as it's prescribed (every day around the same time of the day and not missed a dose? If so, there should be no reason why it will not be effective in preventing pregnancy.

When I breastfed my kids (my first for 12 months, my twins for 14 months), I didn't get my period until 1 - 2months after. You could be experiencing the same.