If i had sex the day before my period arrived (i'm on the birth control pill), and then got my normal period, am i definitely not pregnant??

Unlikely: Pregnancy occurs with fertilization of an egg by sperm and subsequent implantation of the fertilized egg into the uterus. Release of the egg and optimal time for pregnancy occurs near ovulation which is at the mid-point of the menstrual cycle, typically 14 days before the start of your period. Once the uterine lining is discharged (your period) there is no place for a fertilized egg to implant.

No guarantees: Pregnancy has been documented after unprotected intercourse on any day of a woman's cycle, including during menses. It is less likely near or during menses, but there are no guaranteed "safe" days. Women with irregular cycles are more likely to have unpredictable egg release dates than those who are regular. If you are not ready to be pregnant, use something.