What age is your bun? Has eating/personality changed?
I found this that might help -
http://www.bio.miami.edu/hare/poop.html
Runny Stool: When Things Go Wrong
# Diarrhea
True diarrhea (unformed, watery feces) is relatively rare in adult rabbits. It can be caused by intestinal parasites, such as coccidia (Eimeria spp.), roundworms, tapeworms, or anything that inflames the intestinal lining. Your rabbit-experienced veterinarian will be able to examine a fecal sample (you should probably provide the vet with a bit both fecal and cecal pellets) to examine them for signs of parasitic infection.
True diarrhea is more common in baby rabbits. One of the most common causes is coccidia, and if you see runny stool in your baby rabbit, you must get him to the vet as soon as possible for treatment. In a baby rabbit, dehydration caused by diarrhea can rapidly result in death. It is wise to consider runny stool in a baby rabbit a true emergency.
# Unformed Cecotropes: Mushy or Runny Stool
The cecum is a delicately balanced ecosystem. If the intestine is moving too slowly, or if the rabbit is getting a diet too rich in digestible carbohydrates and too low in crude fiber, the complex population of bacteria in the cecum can become "unbalanced." This condition, known as CECAL DYSBIOSIS. Simply put, cecal dysbiosis means that the beneficial bacteria (e.g., Bacteroides spp. and a variety of others) are outcompeted and outnumbered by less desirable inhabitants such as yeast (a fungus, usually Saccharomycopsis sp.) or even very harmful bacteria such as Clostridium spp., related to the ones that cause tetanus and botulism.
A rabbit suffering from cecal dysbiosis will produce cecotropes that are mushy, pasty or even liquid. They are usually quite foul-smelling, and often stick to the bunny's back end in great, nasty clumps. These unformed cecotropes are not a primary disease, however. They are a symptom of a disorder somewhere in the bunny's system. In order to cure the runny stool problem, the underlying cause must be addressed.