Not really sure what happened, but a LOT of things happened.
First, when my opponent sent in Landorus, everything disappeared from the screen except for the rocks and the screen was reversed. I could still select my own moves though. When I KOd Landorus, he sent in Pawniard. I then saw a picture of Pawniard, his HP bar, both Sawsbuck and Pawniard's names and my Swords Dance boost (he wasn't affected by the +2). The Image above shows this moment.
After that (after the screenshot), when Sawsbuck fainted, I had 4 choices: Landorus, Gloom, Bouffalant, and Pawniard (not his other 2 Pokemon). When I clicked on Pawniard, I sent out Shellos (in the 4th slot as you can see). After Shellos fainted, when I clicked on Pawniard again to see what happened, and I could no longer battle.
I doubt this is a programming glitch, but it happened.