Basically, no? She's not pissed. She's concerned for your health, but she doesn't mind the pc fucking Kas.
I guess I remembered wrong but that was pretty sad to read. Poor Brienne. Now I can never finish the Kas romance route. I mustn't make best cow girl sad. Speaking of, why doesn't Brienne have some kind of rancher set, with a lasso and trample abilities? Then the cow girl can be a cowgirl.
She (and also other demons) are supposedly capable of transforming at will. They mostly don't because the form you see is most likely the form they most desire to have, but Kas, at least, is extremely skilled with transformations, so she can just keep it however she feels like.
I think she can make her pussy tight at will if I remember right.