sperm can live in you for...3-5 days..i think, someone correct me if i'm wrong here, and the egg needs to be fertilized within 48 hours after it is released(again, correct me if i'm wrong) so, having sex the day before you ovulate..unprotected..probably wasn't the best thing to do...
If you didn't want to become pregnant, then YEP, you should worry. A lot.
If it has been less than 72 hours since you had sex, then you can go to a pharmacy and get Plan-B. Though, since you were supposedly a day before ovulation, that might not work since Plan-B prevents ovulation... and it may be too late.
All you can do right now is take Plan-B if you are under 72 since unprotected sex. If not, then you are just going to have to wait until your period and then test, especially if your period is late or light. You CAN get a period and still be pregnant. My mother was 3-4 months pregnant in college once and didn't even know it because she kept having a period. She aborted, but that's not the point.
Sperm can live in the body for up to a week (seven days). So, you can get pregnant at ANY TIME in your cycle if you do not use protection. There is even a chance to get pregnant if you are using protection. So yes, you should be worried. Please educate yourself before you have sex!