Ok this is what it says on Guinea Lynx...
DIARRHEA
Diarrhea is especially serious if accompanied by the pig looking ill and sitting with its coat puffed up: get to a vet. A black, foul-smelling watery mess indicates a very serious intestinal problem (bacterial infection, eating spoiled/moldy hay or vegetables). For diarrhea resulting from antibiotic use (killing off intestinal bacteria), consult your vet, who may switch to a different one and prescribe an intestinal bacterial supplement (a probiotic).
Milder forms of diarrhea (too many fresh fruits or vegetables or a change in feed) are also serious, require immediate treatment (generally replacing fresh vegetables with lots of timothy hay), and with no rapid improvement require a trip to the vet. Severe diarrhea can also be caused by E. coli and giardia. Your vet will do a fecal culture to identify the problem and prescribe appropriate medications. Read Seeking Advice On Diarrhea at Guinea Lynx Forums for additional advice.