[targonrotts]

she might be a bit uncomfortable. is the class too long? if so, you might want to give her a break every so often.
also, she might be going thru some growing pains too, and this might also add to the discomfort.
many times, as they get older and start to muscle in the rear some, they do better. keep her thin, take her swimming to help develop her rear and add stamina. exercise with moderation.
have your vet recheck her. she may be a candidate for surgey if very severe.