You can get pregnant anytime you have sex - even if you use a condom and if you are on birth control. Using a condom (or being on birth control) reduces the risk of pregnancy - but the only 100% way of preventing pregnancy is complete abstinence.
Improper use of a condom (like using one that doesn't fit properly, using a condom past the expiration date, not wearing correctly (which can cause the condom to break), or not using it from the very start of sexual activity until after ejaculation can increase the risk of pregnancy.
If you think you may be pregnant, take a home pregnancy test. You can get pregnant even with proper use of a condom.