pawn glitch


Looks like your opponent played the perfectly legal, special chess move called en passant.
This position was taken from your last game which confirmed this:
I recommend checking out this official article which describes en passant in detail with several examples: https://www.chess.com/terms/en-passant
In short, en passant could be thought like this: if a pawn moves two spaces forward in one move, then it can be captured from the opponent's pawn as if it had moved only one space.