Hello and thanks for researching this very important question!
The Guinea Pig immune system is quite fragile...eating the wrong foods, or not eating proper portions of the right foods can damage their ability to fight off infection on a daily basis. "Minor" respiratory symptoms like sneezing can progress to worse more life-threatening problems, so you are wise to seek treatment ideas now.
Guinea Pigs can be sensitive to all the things that bother people: candles, air fresheners, dust, mold, pollen, cedar, pine and other aromatic chemicals. Make sure this little one is being housed in a room where chemical cleaners and fresheners are not in use.
Guinea pigs are susceptible to a number of cold viruses and can become ill quickly from eating a diet that is lacking in vitamin C. Packaged pellets are a good source of protein, fiber and other nutrients, but Vitamin C is very fragile and decays quickly under warehouse conditions. Never handle your pet when you have a cold...GPs can pick up human cold viruses easily!
Many dog-and-cat medications are toxic to GPs, and can make them sicker (ex: amoxicillin). These pets should not be treated by veterinarians who are not fully familiar with their unique physical needs.
What to do at home:
1) Please go to the pharmacy right away today and get a pediatric vitamin C supplement (liquid)...give 25mg today and then 20mg every day to maintain proper immune function.
2) Make sure your house is warm enough for a GP...raise the room temperature to 80F degrees while she is sneezing, then you can keep it at around 75F degrees after recovery.
3) Also get some jars of baby food fruits and veggies so that you have some soft foods on hand in case she stops eating and you have to keep her energy and nutrition high. You can also mix the regular pellet food with warm water to give with a spoon or dropper if you don't see chewing.
4) Guinea Pigs can get fungal (yeast) infections in their respiratory tract and lungs...a dietary supplement such as Bene-Bac or Probiocin (found at better pet stores, Online, or at veterinary offices) can help combat this type of microbe, or prevent it from getting a hold in an already ailing pet.
If this little one does not show any significant improvement today, or gets any worse, please seek veterinary help right away...GPs are very fragile once they become sick and stop eating. They can look fine one minute, then perish seemingly "in an instant".
These links will help you find qualified medical assistance (not all medicines are safe to use in GPs, so you want to have your tiny friend examined by doctors who know how to treat exotic pets):
http://www.cavyinfo.com/html/vet.htm http://www.aracnet.com/cgi-usr/seagull/vetfinder.cgi http://cavyspirit.com/vets.htm
If you need additional support at this time, please "Reply", otherwise I thank you in advance for your "Accept", and would like to know how things are going.
Retired Veterinary Technician
Dog, Guinea Pig, Hamster, Gerbil breeder / Reptile Keeper / Bunny-Ferret-Exotic Specialist