What Causes Absence Of Periods While Breastfeeding?

HI doctors, my wife is breastfeeding a 6 month baby and after giving birth till date she has no period at all..but on 04/03/2015 we both met and i ejaculate inside her..and i given unwanted 72 after 48 hours,my doubt is as follows:- #1=is there any side effect to my child by drinking milk after given this tablet? #2=could she get pregnant without having a period..?(she had no period from last 6 months) #3=is there any abnormal side effects if she has no bleeding at all? plz answer any gynecologist and pediatrcian,. THANKS in advance