Olive was a little wary about cutting into her bear, afraid of what she was going to find. The beanbag was (initially) less gruesome than she had feared. After seeing what happened the first time she tried the spell (which wasn't a very great first attempt), she got v pale and a little green around the edges. Even after her bear returned to normal after the 5 seconds, she was REALLY regretting breakfast.
This was the first class where she didn't make much of an attempt to get much better than her initial attempt. If He-Carrow insisted she try to do better, she would have ... but she also might have lost her breakfast. Either way, she definitely did not manage to perform it perfectly during the class.