When Buffy died in season 5 who became slayer?

Nobody, because since Buffy had already died and activated another slayer she had already passed the slayer torch to Kendra whose death activated Faith. It would have taken Faith's death to call another slayer, not Buffy's.

no one, because Buffy had already died once, and for the first time she died another slayer replaced her

Buffy died and Buffybot became to slay because she died but Buffy returns :)

It's odd though, wouldn't you expect that once Buffy had died and passed her slayer torch that she would be relieved of her slayer powers/duties?

No one. Buffy questions this herself in Season 6, wondering why she was brought back and how it was even possible. Also, in the beginning of Season 6 it is explained that no one, in the world or the underworld, can know that Buffy even died.

It can also be inferred that maybe no new slayer was called upon because the world only needed Buffy, which is why the Scoobies were able to bring her back.

Nobody

no one just like mermaid tail said