One of the reasons I like the Bella character is her ‘strict’ character. I also like her sense of humour, which it always irritates me the MC doesn’t get. I think of her not as someone who is crazy, but as someone who is broken. Something happened, presumably with James, that meant she shut down her emotions as a defence mechanism, and she’s never truly dealt with the trauma.
That all said she can’t of discovered the panties until after she helps clean the MCs room. I think she was probably more angry with herself, that she knew she shouldn’t have ’cheated’. She was probably embarrassed that she’d let herself be vulnerable, that she’s taken a step she ‘knew’ she shouldn’t because no one is as ‘good’ as James.
In some ways the Cathy reaction is odd when she has essentially given permission for the MC to date Jill. I don’t think she’s surprised MC is having a relationship with someone else, more the judgment of the MC for carrying out that affair in her, another person he is intimate withs, car.