Your guinea pig could be shivering for a number of reasons, but one of the main causes of this behaviour is fear. Is there anything that could be frightening your guinea pig? Is there a rat nearby? Has a cat or dog been staring in through the bars of your guinea pigs’ hutch? These are all nasty experiences for a guinea pig, so be sure to do a thorough security check of the hutch and run if you see this behaviour.
Your pet could be shivering because it's cold
It could also mean that your pet is cold, as this behaviour is often seen after a bath. Your guinea pig should only ever be damp when it’s gotten out of a bath. Damp guinea pigs should be gently gone-over with a hairdryer. Be careful to keep the barrel moving, have it on a low heat setting, and avoid the guinea pig's’ ears as they burn very easily.
Shivering could also be indicative of illness. If your guinea pig is listless and shivering, then it is very likely that it needs medical attention as soon as possible.