Bella doesn't believe in cheating on school work, but then she took a Spanish test and everyone else was cheating, so she did it, too. Now, she tells herself that cheating is usually bad, but sometimes it is O. K. This is an example of:

One way of calling this is to say that it's "cognitive dissonance".

Cognitive Dissonance is the state in which one believes something, but also allows the opposite view (so she believes at the same time that cheating is bad and that cheating is good) or when one believes something but acts differently.