Analysis of a position-Why is my move so bad? I am trying to understand this position a bit better and would appreciate some help in doing so. I am black and it's my turn... 

I played the move ..c5. I don't have a deep understanding of this specific position, hence my question (I'm ELO 1700) but my thoughts were along the lines of preventing white to get Bb4 in and at the same time gaining a tempo by kicking the knight. This then also allows me to bring my knight back into the game without obstructing any of my rooks. 

For some reason this move was extremely bad (listed as the 10th best move with a relative score of 2.0 points lower than the best move)

Can someone please explain to me why this move is so bad? 

[title "Black to play"]
[fen "1n1qr1k1/pr3ppp/2p2b2/8/2PN1Q2/2BP2P1/PP2PP1P/3R1RK1 b - - 0 20"]

1... c5
The reasons for not playing 20...c5 are mainly tactical. 20...g5 21. Qg4 h5 22. Qxh5 Bxd4 23. Bxd4 Qxd4 24. Qxg5+ Qg7 wins a piece for two pawns. Since you are already down three pawns for nothing, it won't do to just sit around and wait.

[fen "1n1qr1k1/pr3ppp/2p2b2/8/2PN1Q2/2BP2P1/PP2PP1P/3R1RK1 b - - 0 20"]

1... g5 2. Qg4 h5 3. Qxh5 Bxd4 4. Bxd4 Qxd4 5. Qxg5+ Qg7