Not sure why it started at the 12th move, please back up to the beginning if you don't mind, thanks.
Lost a Slav game, thoughts on my play

3. a4 is not in the database, he seems to be wasting time on the queenside, you should develope the kingside.

From your moves and comments I gather that you have been playing the semislav and in particular the Meran variation with black. However, trying to use the same setup regardless of what opponent does is a doomed strategy. After 3. a4 you should forget about Meran ideas and try to come up with a plan that better answers the requirements of the current position.
In particular, 7. dxc4?! just seems to lead to an inferior version of QGA where the knight has been placed to d7 prematurely. After 4... Nbd7?! 5. cxd5 cxd5 6. Bf4 might actually be quite good for white because the Nd7 really should go to c6 here. So, 4... e6 should be more precise way to reach the game setup. However, because white lost a tempo with 3. a4 also weakening b4 you would really want to play more aggressive than 4... e6. 4... Bf5 placing the bishop outside the pawn chain before playing e6 looks quite interesting. Playing 4... dxc4 also looks logical. Then 5. e4 e5! should be just fine for black but I'm not sure what to do after 5. e3. 5. e3 Be6!? ala Fiveofswords looks quite tempting but I'm not 100 percent sure that this is entirely sound (it might be). 5... e5 might also be possible here altough probably not as efficient as after 5. e4.
11... e5 looks like a normal move to me. Certainly playing e5 is a much more natural idea than playing b5 here.

It's not a slav, it's a semi-slav as Shakaali pointed out. I don't know how to play a semi-slav defense, but I'm sure your moves 8,9,10,11 are very poor. It takes the Queen four moves to go from d8 to b8 (where she doesn't belong... I think she belongs on c7 -- which should have taken 1 move) meanwhile in response to each meaningless queen move White tidies up and betters his position. I think the game gets away right there. By move 11 White has no worries while Black has no center and no space. 15.e4 is white's natural move to uncork a kingside attack to accompany his queenside pressure -- and black, pressured on both flanks and without any space to work with, crumbles.
This can't be the right way to play a semi-slav. I don't understand choking the light squared bishop with Nbd7, and e6 for no good reason, but then, I don't understand the semi-slav... will someone explain what the heck Black's typical plans are in the semi-slav -- it seems like it's built around sitting there wating to be attacked.

will someone explain what the heck Black's typical plans are in the semi-slav -- it seems like it's built around sitting there wating to be attacked.
There are many typical plans depending on the situation of course but the freeing pawn break e6-e5 opening the light squared bishop is one of them. I wonder if blacks position is really that bad after 11... e5!?
Black has wasted some moves with his queen but on the other hand Bd2 and b3 contribute very little if anything to whites position. By the way, 9. 0-0 instead of 9. Bd2 looks more usefull.
Just lost a game playing the slav against the Queens Gambit, and my opponent threw a twist in that seemed to throw me off my game. All thoughts/comments appreciated.
I'm pretty sure where I went wrong, but how would you have played these positions differently?