 [ Symptoms ] what causes low blood pressure [ Disease ] hi if like my wife you keep a diary of you periods as a way of tracking them from this you can work out things like when your last period was and from this when it should have been ok so far if your is just a 28 day period then about day 14 this would have been your last ovulation day give or take a day so if you had sex around two or 3 days before that date bingo you can now put your finger on who is the daddy good luck