She can either have gone off her pellets or it can indicate the bedinning of tooth trouble with molars (back teeth) overgrowing.
I would recommend to try another brand of good quality pellets or muesli first. You are obviously not from Britain but haven't given your country, so I cannot recommend an alternative variety.
Try and see if she is eating by trying to get a mild antifungal/antibacterial mouth gel from the pharmacy; you (or your mum) will have to say that it is for one of you. They are usually available over the counter without prescription. If she perks up with that, it can indicate a mouth rush or tooth trouble.
Please weigh her daily on the kitchen scales (as you should with all guinea pigs that are not 100%) and write down her weight to monitor her food intake. The weight can vary about 30g/1 oz; you are looking for a constant downward trend.
Make also sure that she has plenty of good quality timothy hay and is eating it. Ideally, it should make up to 80% of her daily food intake. That will help best to grind her teeth down and keep the guts moving.