Guinea pigs are good at hiding illness until the last moment.
This site lists illness signs and suggests a vet visit is the best way to go
http://guinealynx.info/emergency.html
"Guinea pigs very seldom get over an illness without help and can decline EXTREMELY quickly. While it may appear your guinea pig has "just" come down with something, he may have been concealing symptoms for some time, a behavior which puts additional stress on the immune system. Concealing illness is a survival mechanism in the wild, where a lagging pig would quickly be picked off by a predator.
For these reasons, if you suspect your pig is ill, it is always best to take it to a vet. If you have more than one pig, you might take along a healthy one so the vet can compare them (unless it has already been separated from the sick one to avoid passing on an illness to your other cavies). "
Hope this helps you!
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