What Causes Delay In Menstruation Other Than Pregnancy?

hi doc...my gf had sex with me on 7july2010 and then she had took i-pill.after that we had again sex on 16july 2010 with protection.she got her period on 14aug2010.and on 3rd and 4th oct 2010 she took 2 pregnancy test.both are -ve.
is she pregnant??why is she not getting her period?is there ant side effect of i-pill??she is taking MEPRATE as per doc suggestion.when should she expect her next period??please reply as sson as possible...