Ds1 did that after his little brother was unwell. I just kept going through with him how important it was to tell the truth and if he was always unwell we'd have to analyse his food or take him to the doctor etc. It took maybe 2 weeks to stop. He didn't actually have butterflies or anything. His was about finding a way to opt out of something he didn't want to do.
Have you talked to her about what butterflies are? How other people feel like that too? Does she feel her heart pounding etc. We were leant some kids books about 'when I am scared' and one about scary noises. Just to familiarise the kids with the fact that everyone has worries etc and things that scare me don't scare you and vice versa.