She
is camera shy, Sayu has talked about it before and you can see it in the backstage footages in dvd magazines that someone(more often Sayu or Reina) has to practically hold her in place to get more than one sentence out of her when you put her in front of the camera alone.
She's been improving a lot in that area though, in this particular case anyhow she was fidgeting and giggling because Haru is trying to tickle her sensitive neck
I guess she doesn't find it much fun being in the receiving end lol.