Yeah, they changed her relationship to Tyrion quite a bit, which now creates problems in season four, as they need to make it go south and make it happen fast. We left her protective of Sansa, with things going okay between Tyrion and her, and all of a sudden she's jealous of Sansa and seem to think Tyrion might have a genuine interest in her, then in the last episode Tyrion pretends he's never loved her and always saw her as just a whore and Shae exploded in tears that were either Sibil giving us some very bad acting, or Shae pretending to cry.... in essence Tyrion broke up with her to be able to send her away to Pentos (but it looks like he was betrayed by either Varys or Bronn). That was... not very good, regardless of the fact it's also very different from the books.

Her acting... well, it's hard to tell if she's bad or if she means to portray Shae as a phoney. It's very hard to tell as the actress isn't terribly consistent in the way she plays the character from scene to scene, even when "no one" watches her and her true self should be transparent. It's like the actress doesn't quite know herself where Shae stands, or hasn't the skills to convey it properly. Through a lot of season 3 and those two episodes, she's been mostly fairly irritating.