Did she piss someone off?

Actually the writers did it to suit the actress's style. Apparently. This gets a mention in some of the commentaries and Harmon actually sounds fondest of her when he talks about the actors.

They more or less try to play off the change as Britta being unaccustomed to letting down her guard due to her weakness of cutting and running - we're supposed to realize (I think) that she cuts and runs so much because she can only keep up the cool facade for so long.

I like her more in season one though, Britta is always adorable but I miss when she was a smart person I could relate to instead of a bimbo who can't see herself through a clear lens.

"Ah. Multiple exclamation points, the true sign of a deranged mind."

I don't think it was to suit her style. If anything, they did it mostly as a response to fans who hated Britta in season one. Harmon has said that the fans didn't like that they were basically being told how smart and cool this character was, and to like her and root for her, so they went the opposite direction and called her "the worst."

And it worked. She's become the MVP of season three after being fandom's least favorite in season one. Unfortunately, I LOVED her season one incarnation--Britta was very real and relatable back then. Now they've ruined a pretty great, unique character by dumbing her down and ragging on her constantly. I agree that she's still adorable but I miss Original Britta.