Dear dimples
It is possible to get pregnant anytime you have sex. Condoms are not 100% guaranteed, and ovulation does not always occur when expected.
If ovulation actually did occur (and only occur) on 'ovulation day', then you cannot be pregnant. However, ovulation can come late. If it was a week late, then your fertile period changes accordingly, and you would have had sex in fertile days.
The idea that ovulation takes place mid-cycle is only true for a regular 28-day cycle. Ovulation actually occurs a fortnight before the next period, rather than a fortnight after the previous period, although this is the same thing with a 28-day cycle. Or more correctly, a period occurs a fortnight after ovulation (with no implantation). So if ovulation is 4 days late, so would your period arrive 4 days late.
Hope this helps.