6 wk old Guinea pig w rapid breathing

Question
I recently bought 3 guinea pigs for my daughters on Sunday from a local pet store. One GP is 12 weeks old and two are 6 weeks old.  All seemed healthy until Wednesday afternoon w the 6 week old suddenly showing rapid breathing. No discharge from eyes. Slowly showed signs of low appetite and drinking.  When we purcharsed her from the store, she did have a bite on her ear and the store clerck said the night before another GP bite her.  I am not sure if this is an issue or not. The other two GPs were also in the same cage as her and are not showing any signs of illness. I can see her fast breathing today, but then she became lethargic. I have been trying to feed and water her by hand by syringe and she is only intersted in water. At first she would take some hay, but within 6 hours now is rapidly decreasing her interest in food. Her only symptoms are the rapid breathing, lethargy, and decreased appetite. She can walk fine and from what I can see there are no signs of broken ribs or limbs. When she is trying to breath, she is softly squakling. She is in a cage with the other 2 GP's which are not showing any signs of viral infection. I am very concerned with her rapid change in her body appearance looking very thin in her pelvic area. Thank you for any advice and time that you could help with.

Answer
I don't mean to get on my soap box again but this is exactly why people should take the extra time to find a breeder rather than a pet store.  While piggies wean themselves from mom @ approx. 4 weeks, I never sell mine until they are 7 weeks old.  A breeder will know all about the piggies that you are getting and there would be none of this "oh yea! she got bitten on the ear last night".  Save your self and your child the regret and tragedy and make you next purchases from a breeder.  The cost may be a bit higher in dollars but there will be no emotional
surprises and a happier turn out in the long run.