When Will My Period Return After Taking Primolut-N For Irregular Periods Following Childbirth?

My son is 3.5 months old.. delivery via c section..he is on mix fed i.e bf and top feed.. after post parturm period I get my 1st and 2nd periods within 2 weeks gap means irregular periods.. 3rd period did not come after passing 1 month.. I this duration I met my husband under protection..mi blood beta HCG value on LMP 31 comes 2.3.. urine test on LMP 34 and 36 also comes negative.. Dr advice me to take Primolut N tablet 3 time a day for 5 days.. when I will get my periods back?