Chess Puzzle

This is a position from my game from yesterday's u160 County match against Shropshire (I have turned the board round so Black is at the bottom of the diagram, 'cos that was the way I was playing):

White to move; I expected 12. Qd1 and while I was pondering whether then to play safe and retreat my knight to b7, or play sharply to c5; my opponent took off the knight: 12. Qxa5.  I must admit for a moment I was shocked, had I made a blunder?

Black to move now: what is Black's next, key move to prove to White that Black has compensation?

Phil.