When exactly was she acting bitchy? Those are all just rumors (that she was a bitch to work with, that she was difficult on set).... Really, I hear nothing but praise for her from her colleagues and nothing but praise from her for BTVS. I understand that she wanted to rest a bit from it, I mean she was working REALLY hard on it for 7 years, so I understand her.
I didn't say she was bitchy, I said she can come across as bitchy, and I dont even mean her I mean the public perception.
There are numerous quotes from her co-stars that imply coldness/distance/bitchiness.
Lots of her co-stars also describe her in the same way "such a professional, always knows her lines" - when you have the same line repeated over and over by multiple people, it also adds to the aura of bitchiness, it began to come across as not actually having something nice to say, passed a prepared statement.
She also basically had a stock answer for talking about Buffy, up till recently she's always described the show in the same way, there's absolutely nothing wrong with wanting to get some distance between BtVS and the rest of her career, but it did lead to a aura that she was cold/bitchy.
Recently that has changed, I think since she's started promoting The Crazy Ones she's decided to be more open/personable, look at press stuff for Ringer and TCO and she is completely different, and is very different when talking about BtVS.
Around the same time Alyson gave an interview that didn't come across so well for Sarah and she joined twitter, and since then she's been very embracing of BtVS and talking about BtVS and interacting with her fans about BtVS.
Personally I think Ringer was a wake up call that fans aren't gonna follow you everywhere if you do crap shows, so she's trying to get fans on board with TCO, which is great cuz the show is all right and she's become a lot more likeable in public perception.