What happened to my game?

So, I was playing with my Doppelganger on Normal mode and about to touch the giant rock to face the Sentinel, and I didn’t wanna have any trouble, so I had my brother’s level 50 Wilhelm join the game to easily kill the thing. And then it happened: my game went negative, everything was really white or black, dropped items had a weird digital look to them (not the actual gun, the little square informing you of its stats was like that), there was no listed price for anything, my subtitles were a dark orange and I have no idea what clumped up language it was, my brother’s Wilhelm’s guns, when swapped, had Eridian text instead of English, just, wth happened? I turned off my system and played the game again and everything’s normal but…what?