I don't see it like that.
For me it's a natural progression.. treading water was the middle of the season up until he heard her "confess" that she remembers the shooting (and that also means Castle's I Love you) and the middle season made me cool off considerably when it came to Castle.
Now however things are in motion since last week and it feels natural to what they're going through, especially Castle. He feels betrayed by the very person he dived in front of a bullet to save her.. something he'd only do for his mother and daughter. Now he "knows" that either she's doesn't feel like him (a huge bummer) or that she had a "Yeah right.. falling in love with a playboy writer.. good joke" and it pissed him of to no end.
Castle is prone to overreaction and this is a major one.. he doesn't know how to deal with that properly (sane and rational adults would confront each other and lay it out in the open by talking it out) so he overreacts and reverts to his old playboy ways where nothing could touch or hurt him.
However he's hurting and no matter what he says he can't turn off his feelings and it will boil until the inevitable explosion which i guess will happen in the season finale (and i'm pretty sure because i've read an interview recently with the exec producer where he flat out said that in the season finale they will lay out the cards and reveal everything).
So no.. i'm definitely enjoying this seasons' end. Prior seasons had time going for it.. it is conceived as a big love story and you don't hop into a relationship after a few months. These things develop slowly especially with all the baggage involved (and the need to extend it a bit for TV drama purposes and put in an occasional wrench like bad timing or being in a relationship with a different person) but there came a time when it became a bit stale or ridiculous and that was 4th season for me.. after the explosive season 3 finale i was on edge to see how it would develop and when she claimed she doesn't remember i got a bit mad because pushing the reset button this easily was cheap but the show surprised me and threw me for a loop and i was content. Middle of season kinda lingered on and i got a bit bored but the end picks up tremendous speed and i'm happy.