She didn't come across like a bitch at the trial because she knew the cameras were on her, and anything that happened would definitely be televised, as the trials always are. Just like Antony Cotton, who'd be truly vile in camp, come down to do a trial, and be all sweetness and light cause he knew he was on TV.
When Lucy was at camp though she was, getting all arsey about Amy not sharing gum with her. Did she really expect to receive gum from the woman she's been treating like dirt for a week and a half. I don't blame Amy for not wanting to share with her.