Hi & Welcome.
Positnor-2 is not known to cause harmful effects in the suckling babies.
The post coital
contraceptive pills like Positnor-2 are considered to be safe during
lactation (
breastfeeding). It does not adversely affect the composition of the milk or its secretion. It also has no adverse impact on the growth and development of the suckling babies.
Withdrawal method is not an effective method of
contraception, as its failure rate is about 25% or more, and the fact that the precoital male secretions contain sufficient number of active and motile sperms and also the Positnor-2 is only 85% effective in preventing unwanted pregnancy the delayed period could be due to an early pregnancy.
The other causes of delayed period include hormonal imbalance, undue stress, emotional upset, irregular dietary habits, obesity, certain underlying medical illness or diseases of the ovaries or uterus.
So, get her home pregnancy test done first then consult a gynecologist.
Since you do not wish to have your 3rd child at the moment, the best form of contraceptive can be non-hormonal like
intrauterine device like copper T if the pregnancy test is found to be negative.
Hope these answer your query.