My 4 year old went through the same thing with her dance class - and I'm relieved to know that she's not the only one! About 3/4 of the way through the lessons she stopped dancing, just refusing to participate. This continued for 3 weeks in a row. She'd go through the motions of getting ready for the class, getting dressed and was excited about going, and then we'd arrive and the others would start class and she sat back and watched. When it was time to change shoes for a particular dance, I'd change her shoes so that she at least felt like a part of the group. Afterwards I'd ask her why she didn't want to dance and she's inevitably reply that something hurt, and then the last time she just replied "because". I told her that I understand that dancing can be difficult to do at times, and that if she wanted she could stop. She explained that she wanted to dance, but that she just didn't feel like it. By week 4, I was ready to pull her out, and she stood up that day and had made up her mind that she wanted to dance.
I think kids get burned out (like one mom had said) but I also think that at times it can be overwhelming, and they just need to relax. In my daughters case, she just needed the reassurance that we supported her decision, and that we understood what she was feeling (even though she was very vague in her descriptions of what she was going through) but the fact that we were there for her made all the difference.
I hope this helps! Just remember to be open and receptive to what she has to say, and I'm sure that this will all work out in the end!