I think that point was brought up on one of the talking head shows. Someone on the show said they thought she might be on some sort of drug. But do remember that look of rage caught on camera—so if she was drugged, it kind of slipped at that point.
Casey does have strong control of her emotions when it is advantageous for her that she does. There’s a video of her response when her mother first took the stand. Having not seen her mom for some time Casey was definately impacted by seeing her....but then you see her immmediately shut that down...and begin writing, which I think her writing was simply a means of “escape” during the court proceedings and helped her regain control. It’s quite evident in that video.
Again, if she was drugged before court proceedings, and likely I think, it would be low dosage.