Poor Daisy.
I could be elbow/hip dysplasia or she may just be having an off day, it happens to the best of us. Maybe she felt a little sick or sad for some reason. Maybe she had a bad experience with something that relates to agility or someone that was there? It could be just about anything.

But it is strongly reccomended before you start agility (or any other vigorous sport) to have your dog x-rayed & a full exam, to check for elbow/hip dysplasia or any other problems that may affect the dogs mobility. As well as a full blood panel to rule out any other medical problems.

Is this the first time she's acted like that?
If so I'd bring her to one more class, if she's still acting the same I'd get her to the vet for a check up.
Does she have any symptoms? If so I'd bring her to the vet before you go to another class.