I'm sorry that your question wasn't answered in a timely manner. Diarrhea is uncommon in adult cavies but soft stools are frequently reported due to excess carbohydrates or inadequate dietary fiber and sometimes with vegetable consumption. Some animals have intermittent soft stools without identification of the etiology. Bacterial enteritis is also a common cause of soft stools/diarrhea and there are quite a few bacteria we can incriminate such as Clostridium piliforme, Salmonella typhimurium and Salmonella enteritidis (less common causes), Yersinia pseudotuberculosis, E. coli, Pseudomonas, Citrobacter, and Listeria. We also have to consider the parasites Eimeria, Balantidium, Cryptosporidium, and Paraspidodera.
Regardless of the underlying etiology, diarrhea is a serious problem because hypoglycemia, dehydration, hypothermia and electrolyte imbalances quickly occur. A pocket pet vet should administer warm fluids containing electrolytes and glucose and dietary correction with adequate fiber is of utmost importance. Antibiotic treatment is based on culture and sensitivity of the stool.
The recommended diet for pet guinea pigs consists of guinea pig pellets and grass hay, supplemented with fresh vegetables. Fruits, rolled oats and dry cereals should be offered only in very small quantities, if at all, as treats. Please respond with further questions or concerns if you wish.