Hi and welcome!
How big is the weight loss? It can have a fair number reasons - teeth, but also very likely a problem caused pain In the body; your vet will have to give her a thorough examination. How are her poos?
Please make sure that you have her crucial back teeth checked by your vet, not just the front teeth, as it is usually the pre-molars growing spurs that are causing dental issues in older piggies. Please book her in as soon as you can, especially if the weight loss is 100g and more in a short time.
We have got a piggy savvy vet locator on the top bar for our UK members.
It may be good if you considered topping her up with hand feed, as just watching her eat and chew can be deceptive and her food intake is clearly not adequate.
http://www.theguineapigforum.co.uk/threads/complete-hand-feeding-guide.115359/
With members from all over the world, it is very helpful for us to be able to tailor our advice and recommendations to your location straight away if you added your country, state or (for the UK) your county/city to your details. Please click on your username on the top bar, then go to personal details and scroll down to location. Thanks!