Suggest Medication For Irregular Periods After Taking Positioner

i took the postinor 2 on jan 1, my last period was on the 24/12/10 for five days. i had a brownish bleeding on the 7/1/11, for 5 days. i took two pregnancy test and it's negative on the 20, and 24. is my cycle messed up from the use of the postinor?