It's Normal for Guinea Pigs to Eat Their Own Poop
Have you ever seen your guinea pig dip his head down between his hind legs, then come up for air chewing on something? If so, then you've witnessed your piggy eating his own poops. This is known as coprophagy, or coprophagia. Don't stop your piggy if you see him or her doing this! Although humans might find this odd or distasteful, coprophagy is an essential part of the digestive process and is considered perfectly normal behavior for a guinea pig.*
There are two types of poops. Guinea pig owners are familiar with the dark solid ones often left scattered around the cage. In contrast, caecals or cecotropes are soft, squishy, and tend to stick together. Caecals are noticeably lighter in color, and have a green or yellowish tint to them. These are rarely seen because pigs often eat them directly from their bottoms, from 150-200 times a day. Since plant matter is not completely broken down the first time around, guinea pigs ingest caecals to maximize the nutrients they can derive from their diet - similar to how cows chew their cud. Caecals contain essential vitamins, such as vitamin K and B-complex. They have twice the protein with only half the fiber of the harder poops, thus nutrients are more easily absorbed. This digestive process is known as cecotrophy, hindgut fermentation, or pseudorumination.
Caecals also contain "good" bacteria that can help restore an ill piggy's intestinal flora. As GuineaLynx says, these feces are the best ones to feed a sick guinea pig on antibiotics to reinoculate good bacteria into the digestive system. The problem is that antibiotics are necessary when your cavy is sick, but kill both good and bad bacteria. Mixing a homemade "poop soup" is a natural probiotic solution to restore the gut flora. To do this, take poop from a healthy guinea pig and mix it with warm water to form a soupy liquid, which can then be fed via syringe to the ill guinea pig. The more solid poops can be used, but do not contain as much beneficial bacteria. Boars (male guinea pigs) with impaction also benefit from poop soup made with the caecals they expel, since they do not always ingest them after removal. Think of it as sort of a dietary supplement to remedy a vitamin deficiency.
Sick guinea pigs have been known to steal caecals straight from other pigs bottoms. When Belka was recovering from her gastrointestinal upset due to an antibiotic intolerance, she was often seen roaming around the cage sniffing at and eating other pigs droppings. During this trying time, it was notable that she seemed to prefer hunting for choice poops at mealtimes rather than compete with the others for fresh vegetables. On a few occasions, she was even observed harassing poor Peaches by nosing forcefully at her rear when Peaches was trying to do her business in a quiet corner.
In short, eating poop is essential to your guinea pig's health. The next time you see your piggy curling into a ball, don't be alarmed - you'll know what's going on!
*Interesting fact: Cavies aren't alone in that matter. Other house pets that are known to engage in coprophagy include rabbits, hamsters and chinchillas.