Recent Win B06 Modern

A Modern game I just finished. I made some mistakes and a blunder. In the end tactics saved the game. On move 16. exd4 my Re8 was a mistake. Stockfish says c5. Again on move 17 instead of my d5 I should have played c5. Can someone tell me what thought process would lead me to play c5. I would not ever have found that on my own. What am I not seeing?

White's pawn d4 is pinned and by playing 16... c5 you get rid of your weak pawn on c6. I think you win a pawn in the process.

For the record, let me add that by playing 4. ...Nf6, you actually transposed to the Pirc. (Nothing wrong about that in this position, though.)