Buns can do that for loads of reasons
~Showing distaste, it's a step down from weeing somewhere to show you they are disgusted with you. In this instance it could be if you are not doing what the bun wants you to do.
~It could be a mini binky, like Jim said. Happiness at what you have done.
~I could simply be something like your bun doesn't like the feel of the material she is standing on and flicks her feet to get rid of the feeling.
It could be anything, but she's not too distressed (if she is distressed) so don't worry too much. Chances are you are seeinga happy bunny.