I've come across something similar. I know of a bunny that was too fat to clean his bum and eat the cecotropes/cecal pellets (a type of poo that they are supposed to eat, as it contains vitamins and other good stuff). They would mush all over his bum, and form a pocket around his entire waste-generating region. This poo crust kept the urine and fecal pelets (not the poo that should be eaten) next to the skin, and irritated it greatly. He needed a bath daily to get this poo crust off and clean the skin. I was using Udderly Smooth, a type of cream people use for their hands but it's made for cow udders, to soothe the skin. Another good option is baby cornstarch to keep it dry.
I agree that we need to know more about her diet, as well as whether or not she eats her cecal pellets. They're smellier poo clumps that look kind of like grapes all stuck together, and are shiny and mushy. Fecal pellets are the little cocoa puff looking things (smaller or bigger depending on the bunny's size) and they are dry, not shiny, and should be hard. If she is too heavy to eat these, she needs to be on a diet.
If she eats them fine, but is just sitting in her litter, there are ways to help with that too. First, make sure the litter is good--paper-based or non-aromatic wood based. I like wood stove pellets the best. They're made of pine sawdust that is kiln dried to remove aromatic oils that can bother the bunny. They are good at absorbing odor and moisture. Other good choices are recycled paper pellets, like Yesterday's News, or paper pulp, like CareFresh. Not ok are wood shavings. Not only do they not absorb smell very well, the ones made from cedar and pine can irritate the rabbits' respiratory tracts and cause issues with the liver. Aspen or kiln dried pine shavings are ok, but they don't absorb smell or liquid as well as the pelleted beddings.
I also like to put grates over the litter so my guys don't sit in it. Some litterboxes come with metal grates, but if you don't have them, plastic canvas (from a craft store or Wal-Mart) is great. They can also get sore hocks (foot sores) from stepping in dirty litter.
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( I think I am going to go to the vet..
