From what Matt had heard of Cas’s mum, she wouldn’t waste her working hours calling her daughter when Brittany could just as well do it. Matt was suspicious, he noticed that Cas had been deep in thought, the whole journey into town. Also, Cas had just pulled out her phone when her mum ringed. Was it just a coincidence or was the whole thing staged?
Matt didn’t want to assume the worst but also knew he couldn’t assume the best either.
Since the “emergency phone call” Cas had hesitated - even if only for a millisecond - before replying to Matt’s questions:
“... Uh... No, Brittany called my Mum. She’s still at work. I really gotta go now though.”
“...I don’t really have any time now. You can pick the canvas yourself though. I’ll see you at school tomorrow.”
The worst hesitation was when Cas, the confident and feisty girl Matt thought he knew, said, “...W-well... Um... Maybe its better you don’t come with me ‘cause we still need to get the ca...