Here's a quote on diarrhea in guinea pigs from the guinealynx :

"Diarrhea especially 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."

Do any of those apply to your piggie girl? I really hope she's okay.