What day of your cycle did this happen on? It sounds like you might have been lucky and it happened on an infertile time.
In general, if you have sex regularly during a cycle without taking any precaution, the chances of getting pregnant are about 30%.
If you want to continue to use only condoms, it is important that you have Plan B ready if something goes wrong. Plan B can be sold to anybody over 17, man or woman.
The best is actually for you to be on birth control pils and use the condom.