Chinchillas have delicate digestive systems. It is therefore important to be vigilant about changes in behavior that can indicate digestive issues.
One of the most common chinchilla ailments is constipation. Usually, constipation is caused by insufficient fiber. Constipation can also be caused by stress from new surroundings, a change in food, and lack of fresh water.
Constipation symptoms include smaller, drier, and thinner droppings. If you suspect your chinchilla is suffering from constipation make sure there are sufficient hay cubes in the cage. Also, offer the chinchilla a raisin or two. If they've been cooped up in the cage for the last few days, make sure to let them out so they can get some exercise. If the situation doesn't improve within a few days you should consult a vet.
Another common chinchilla ailment is diarrhea. Diarrhea is fairly simple to detect as the chinchilla's stool will be soft, sticky, and easy to flatten when stepped on. Too many treats or a change in food can result in this ailment. The first thing to do when detected is to stop providing treats. If the problem persists and there's no reason to suspect that the pellets or hay are bad, consult a veterinarian.
If the diarrhea persists for a long time, it could be a sign that your chinchilla's intestine is irritated or inflamed. These things can be caused by infection or the extended use of antibiotics. Along with the continued diarrhea, a chinchilla with enteritis will often lose interest in food and treats. If you think that your chinchilla may have enteritis you should take it to the vet.
Bettina Hansen's (website no longer live)
A New Owner's Guide to Chinchillas by Audrey Pavia