How is this possible?

Did you notice that you were in check from two pieces?  Both the Queen on e1 and the Rook on d2 is checking you.  You can't get out of check by taking the Queen with your Rook because you're still in check from their Rook.  You can't get out of check by taking the Queen with your King because the Queen is protected by the Rook on e8.

it's because white's king is in double check, neither of which can be answered at the same time. one rook protects that evil queen (even if white could take her with his own rook, there is still the other check by the other black rook), and white has no escape squares.

and that omnipaul beat me to it, too.

When your king cannot get out of check by capturing the piece(s) that put it in check, cannot escape to a safe square, or cannot interpose a piece to get out of check, you are checkmated.

thanks

well played