I am sexually active and about a week before my expected period.i got few drops of blood probably dark brown withot period pain.implantation bleeding?

Maybe (not) . . .: Can't really speculate as to whether you're pregnant or not. To save anxiety, just take over-the-counter home pregnancy test available at most grocery or drug stores (at least in the states - not so sure about uk). But since you're worried, that means you really need to chat w/your gp about birth control options. Also need to make your partner wear condom each & every time to lower std risk.