If you are on pills still, do not worry, continue to take them as usual. Anal sex does not result in pregnancy. Even if the precum came out and went into your vagina, the fact that you were 8 days post the start of your period and still on the pills, will not let you be prtegnant. Be careful and judicious in what you do rather than do the act and then fret about it. Behave sensaibly its your life and also in case you do get pregnant you do not have any rights to kill the baby concieved as it did not have any faults of its own, but got life due to your wrong doing. Either follow proper contraception and safe sex practices or abstain from sex altogether.