Thank you for getting back to me. I'll give you some first aid measures in a moment, but at this point, your guinea pig is very ill, and she needs to be seen by a vet as soon as possible. Here is where you can find a guinea pig/rabbit vet:
Since your guinea pig has had diarrhea for a week, she is probably getting dehydrated. Get some Pedialyte (made for human infants and available in discount stores and pharmacies). Use a syringe to put some in the side of her mouth (not straight down the throat - that can lead to aspiration into the lungs). For the first hour, give her some Pedialyte every ten minutes. After that, repeat it once per hour.
Remove all of the food from her cage. The so-called gourmet mixes, which contain seeds and corn, are very bad for guinea pigs. They cannot digest these foods. Give your guinea pig some timothy hay (not alfalfa hay). You can buy it in bags in pet stores. She should have all of the hay she wants. Plain guinea pig pellets should be provided - hay should be the main ingredient, and no seeds, dried fruit, nuts, etc. You can read more about feeding guinea pigs on this site:
Eliminating the mixed feed, providing hay, and using Pedialyte are only first aid measures. They won’t replace a visit to a vet. It's very important to see one quickly. If you have more questions, just let me know by clicking on REPLY. I hope your guinea pig will reach a full recovery once proper treatment is begun.