Hi. my girlfriend's period was due on the 26th and it's the 3rd and she hasn't got it yet. She complains of pounding headaches and upset stomach. We are sexually active and we didn't have sex since on the 24th. On the 27th (one day after period was due) she took a test, it came back negative. Is it possible to be pregnant one week after receiving a negative from a pregnancy test without having sex??