It's unlikely but it's not impossible. Generally what happens is that you ovulate around the middle of your cycle. If you have a 28 day cycle this is around day 14. But if you have a much longer cycle, ovulation is around 14 days before your period. But when you ovulate is different for each woman. The average is day 14 but you may ovulate earlier or later than this.
When you ovulate your egg only lives for 24 hours. So theoretically you have 24 hours in which you can get pregnant. But having said that sperm can live for up to 5 days.
If you are worried about getting pregnant then you either need to look at using reliable contraception all the time or looking up fertility signs (ie billings method)