You usually ovulate around day 14 of your cycle if it is a 28 day cycle. The first day of your period is day 1 so count from there. If you definitely don't want to get pregnant then use a reliable form of contraception at all times.
I found out I was pregnant 3 months ago and it wasn't really supposed to happen then but wasn't a disaster for us. I thought avoiding the fertile week would be ok after previously failing to fall on months before. That particular month I ovulated 5 days later than usual so don't take the chance!