My 4 year old won't participate in group activities?

is it common for the preschooler to suddenly not want to participate in organized group classes or activities? my daughter has tried a couple of rounds of ballet and gymnastics when she was 2 1/2 - 3 years old. no, these are not "academic" kind of classes, just your local community type of classes like what ones that Gymboree holds... all simply for fun, she's even met friends from there and they usually aren't more than 8 - 12 classes at a time, once a week. she loved every moment of them and would get soo excited to go. it's been a few months since we've taken classes due to the winter season, etc.. recently, our 9 yr old neighbor, who is taking rythmic gymnastics, taught her how to use her hula hoop and ribbon and for a 3.5 year old, she's become really good. at first i didn't think to enroll her but she asked me herself "mommy, i want to try 'ribbon classes like megan'" (lol) so i enrolled her in one for 3-4 year olds. we've had three classes and she will not participate. but every single time, she would get sooo excited about it all the way until we walk through the gym door. it's so strange because it's like she's a completely different person. one moment she'll be so excited to go, and then as soon as the class starts, she clams up and hides against me and will absolutely not want to participate. has anyone else been thru this before? she's in preschool two mornings a week and there she is completely fine.