Yes, you should take her to the vet right away. It sounds like she has a URI
Guinea Lynx :: URI
Guinea pigs hide their illnesses very well, but they go downhill fast once you start to notice symptoms. You need a cavy savy vet who will prescribe you safe antibiotics.
Guinea Lynx :: Antibiotics
You can also post on guinealynx and get great medical advice there as well.
Make sure you get a scale and start weighing her. Weight loss is usually your first sign that something is wrong. Make sure she is maintaining her weight. You should be prepared to handfeed her if she stops eating:
Guinea Lynx :: Hand Feeding
Good luck with her and please let us know how it goes at the vet! I hope you have a vet who knows a lot about guinea pigs, because they can be tough to find. Here is a list of cavy savy vets recommended by members of guinea lynx. Hopefully you can find one near you:
Veterinarians - The GLX-Files