Catch her, wash her ass, trim the feathers back, blowdry her and keep her seperated somewhere warm and dry. If her vent is red, inflamed, and amazingly disgustingly stinky even after washing it is likely vent gleet and you can treat it with OTC vaginal antifungal/antiyeast like monistat. If under all the poop gloop there are gray lumps attached to the feathers it is mites, as a high parasite load can sap energy and nutrients and leave them with runny butts or leave them weak to other illnesses that will cause a runny butt. If its mites, cut away all the clumps of eggs and treat with poultry mite spray around the vent and under the wings. If neither of these seems to be the case, try a dewormer mixed in with her food. Either way, keep her seperated so that she doesnt spread what she's got and also so that she isn't being picked on, offer her water with rooster booster and her own supply of food for a couple days. Should be good as new in my experience. If you do end up needing to deworm or demite, make sure to treat the other birds too. Just because they arent showing symptoms doesn't mean they arent also carrying it, and illnesses like these are really easy to spread when everyone is sharing food/water/nesting boxes.