What Causes Missing Periods And Negative HPT?

gud day doc im ronald from philiphines. my gf is delay her for almost 1 week, she taking a medicine pottasium iodine coz she have a problem in sorthroat. why she delay in menstration? but she using p.t the result is negative. is true why she delay bcoz the taking medicne. sorry to my englis is poor. hope u understand . thanks