I've just found Popcorn looking scruffy and with what look an awful lot like bite marks... Will upload a video later on.
Dolly is prime suspect as she's grumpy about her eye drops, as well as having a slight history of being aggressive during introductions to other mice. Cookie has nipped me a couple of times, but she's always got on well with her sister Popcorn.
Poppy definitely isn't happy, but as a temporary measure I don't know whether to put her alone in a cage and keep Cookie and Dolly together or to isolate Dolly and keep the sisters Cookie and Popcorn together? :(
My instinct says Dolly did it, but as there are no marks on Cookie or Dolly it's hard to say for certain. Putting Popcorn in by herself would seem to be the safest option, but I don't know how she'd cope...