My bunnies are very competitive about who owns the living room. Each pair has its own bedroom and we let them run about in the living room on alternate days, which leads to a poop-fest by whoever is out. One of my bunnies is so desparate to mark the living room with her poo that she sometimes leaves mushier poos. Another possibility is that you're used to seeing poo that is a few hours old, rather than a few minutes, so the kind you're used to has dried out a bit more. Rabbit poo is a bit mushy when it is newly formed--it doesn't turn into hard little cocoa puffs until a bit later. Finally, the change in her greens could be to blame. Did you add a particular type of green? Some bunnies can't tolerate some of the greens and may have a certain sensitivity. I know of bunnies that can't handle cilantro, for instance, and it gives them mushy poos.