Hi Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern.
Yes postinor pill can cause
irregular periods. The first time your girl friend had bleeding(which was a week earlier than her normal period) must be
breakthrough bleeding. This type of bleeding normally occurs after intake of
emergency contraceptive pill like postinor. There is nothing much to be concerned about.
She could also have his next period delayed. Delayed periods are common side effect of a emergency pill like unwanted 72. So her periods could be delayed for 1 or more cycles.
Use condom or regular contraceptive pill as method of
contraception in future. Taking more emergency contraceptive pills can cause problems like irregular periods, ovarian hyper-stimulation syndrome & even
infertility.
You should also do a pregnancy test to rule-out possibility of getting pregnant
For more queries consult a gynaecologist
Hope this information helps