So I lose 1 game when I have 0 lp and I get demoted?

You get 3 games of demotion protection upon being promoted a division (10 for tier), and once you've played those 3/10 (won or lost) then it's gone and you are going to be at risk of demotion any time you lose at 0 LP. If your MMR is really good then you might have some protection, but otherwise that is just how it works.

Well I can't help it when people pick champions they've never played before and bring them into ranked, so I guess i'm fucked regardless because I get queued with metamancers.

If you win in a division and then lose after having 0 LP you will get demoted, otherwise without winning you should have no problem.

You're mmr must've been really low so the system figured you needed to be demoted to place in what it thinks you should be at rank wise. It's not a huge deal, but quite annoying. Ive had it happen a few times to me too this season as I'm making my climb. To fix your mmr you really gotta buckle down and win some games, eventually you'll get it back to normal and next time you lose at 0 lp you'll maybe have some protection