Why do you play 3. c5 ? The only time I find myself playing c5 in this opening is if I play Qb3 (e.g. if black has moved his light-squared bishop) and black responds with Qb6.
queen gambit variation

In my opinion 3.c5 is a bad idea. You will have to trade with black's b pawn (or he will). You won't be able to close in black's pieces and make a good hole for a knight later in the game.

3.c5 is a bad move at this stage. W is not ready for closing black in, and the move reduces W pressure against d5. B can counter 3.c5 in various ways, i personally play the very thematic 3...e5!? , putting W centre under serious pressure and getting a free development. The old saying "counter a side attack by playing in the centre" looks very spot on here.
Yeah, c5 there is a bad idea. It realeases all the tension in the center, means that black's pawn on d5 is going to remain strong for the near future, and wastes a tempo. I mean, you're not developing and you're giving up the option of taking on d5 which is often a good response depending on how your opponent plays. c5 might...might be a good idea later in the game, though honestly it doesn't happen that often because it's so easy for black to then break in the center with ...e5.
I was wondering if anyone knew anything about this situation that I keep running into against the queen's gambit declined.
1.d4...d5
2.c4...e6
4.c5...b6
o.O
I usually end up pushing up my b pawn and doing funny stuff with my bishop and knights, btw.