Mythical info:
As far as pvp goes, there is probably a lot of ppl better informed to educate you on that, but basically your toon fights ‘online’ when you control it, and ‘offline’ vs other players - much like when you’re fighting someone. It’s an AI following a script, so if you’re doing pve, and you have bad items/bad AI setup, or a bad build. Ppl will beat your AI controlled char and you will loose rating because of the losses. When you play yourself, you can micromanage your char, so that you will win a majority of the match ups, and gain rating while controlling your char.
Atleast that’s why I think you lost your rating. Could also be that someone just got more points than you have, due to more games than you… Dunno:)