She was going to tell. This conversation was leading to Caitlin telling on Cora about what she thought was a breach because suddenly his sister, who had only just gotten here, had a pet no one knew about. This was a nightmare. He'd never had trouble transitioning in or out of the form, so why now? Why when he needed to be able to shift back and change into something so that he and Cora could spar to throw someone — Caitlin, in this case — off? Why, for the first time since gaining the ability, had Derek had trouble controlling it?
More importantly, he could tell that Cora's willingness to play sympathetic was waning and Caitlin wasn't budging for it, either. When Cora stood, Derek rolled onto his stomach again and sat up straight beside her, looking back and forth between them with probably a little too much alertness for a dog, but he couldn't help it.
He wished that Cora had thrown out his name specifically when she went with saying that her brother was on Defense, because he wondered if it might've won her some brownie points. He and Caitlin got along fairly well and he, at least, thought there was a little harmless flirting going on between them.
God this would be so much easier if he knew that he could trust her. He could try again, to transition out, and as long as he stood behind Cora when he did it, his sister's body would hide anything Caitlin probably shouldn't see, but he didn't know Caitlin that well and he didn't know if he'd end up being her next science experiment if he did. Cora's tone was slipping out of genial and hopeful and back into distinctly Hale and that was not good news.
Try to transition back and explain his way out of it; beg Caitlin not to say anything? Approach her as the wolf and degrade himself a little further by going full on domestic dog on her and trying to beg for human affection? Neither option was very appealing but Cora was becoming less helpful by the second.