why was this a bad move?

i thought my queen sac would win me the game... and it did... but the analysis says it was a terrible move.

why?

20 Qc4?? Qxc4 21 Rf8+ Rd8 wins for black.

And even in the final position 21...Qxd3 22. Rf8+ Qd8 23. Rxd8+ Rxd8 is a rook ending with an extra pawn for black.

yeah. black should be winning

I think tpr's line is clear: 20 Qc4 is a blunder because Black queen can just simply take and then use the rook on the d file to defend against the White rook's check.

If White rook then takes either of Black's rooks, Black would simply recapture for an absolutely crushing advantage.

I remember that Tarrasch wrote in his famous book 100 years ago: „Eine Wendung, die man als Verteidiger und Angreifer kennen muss!“ Indeed, one should know this defensive manoeuver.