- And got the T-shirt
However, what you may be seeing is the result of her biting and scratching herself because she has mites. Unless you're seeing one bite the other, the problem may be self-inflicted. You can post a picture on here and we'll take a look and see if we can figure anything out. If you have trouble doing that (you'll have to host the picture on some other site, and link it here), you can send a PM to lynx and she'll post it for you.
If she is sniffling, she needs to be seen by a vet. A pig can go downhill very quickly with a URI, and the sooner you start treatment, the more likely the pig is to recover.
Unfortunately, you've discovered two of the main reasons why people are so against purchase of guinea pigs from pet stores. They are often sold sick, with parasites, mis-sexed, and the baby females are pregnant. (BTW, you have turned them both over and compared their private parts to be sure they're the same sex, haven't you?) They're also inbred, as breeders that sell to the big pet stores make no attempt to establish and maintain safe bloodlines.
When you get a guinea pig, you have the responsibility of getting medical care for them if they need it. Talk to your mother. If you cannot work something out for now and for the future, I have reservations about your having pets.
Mites are very common. And DO get a bigger cage! Weigh daily right now, watch for scratching, watch for any other signs of illness.