When Will Periods Start After Taking Meprate?

Hi i had i pill on 17th may..my periods came on 25 may which was due then..but my periods dint come this month yet..its around 20 days over.i tested pregnancy home test two times and it came negative..Now my gynae prescribed me with meperate 10mg 2 times a day for 5 days...could you suggest after how much time finishing this medicine..i could have my periods?please suggest..i am very much tensed..do taking an ipill only once in my lifetime (till date) has any side effects on my body?