Could she be pregnant? Many answers very appreciated.

my girlfriend had sex last week. I had already pre-came before penetration, and we used the withdrawal method. she is also on the pill. her period should have been today, but it isn't there yet. she also usually experiences her PMS a week in advance, but it isn't there either. 2 weeks before sex she was always a few hours off when taking her pill for 4 days. her periods are usually constantly the same. help and advice greatly appreciated. thank you.