A condom is the most reliable form of contraception - as long as care is taken not to damage the condom and that it doesn't come off his penis inside you, then you should be protected. I can see how worried you are about being pregnant (the message earlier said you were due for your period on the 18th). As I said in my previous response - clear your mind by taking a test, and maybe you could pop into a local family planning clinic or Marie Stopes and get some professional information about how to protect yourself in future....learn more about your body, how it works, and how you can keep yourself safe. This should help you with the worry in future.