Weight loss is always a concern with a piggie (unless you've been adviced to slim one down)
6 is a good age for a guinea-pig . Is your pig a lone one . Have they recently lost a cagemate ?
Is your guinea-pig struggling to eat , dropping food out of her mouth ?
First query would be teeth. Check the front ones . Is one broken or uneven?
You won;t see the back ones but your vet can put a thin light down with a magnifying glass . If there's an overgrown or sharp spur they might be able to file it depending on your guinea-pigs temprement but she might need sedation.
Are her eyes nice a popping and bright ?Or sunken? Can be dehydration . The eyes and teeth of piggies are very close together , teeth can affect eyes .
Are her droppings normal for her ? Or slimey? mis shapen ?
They can get a yeast imbalance in the gut . Too much sugar / fruit can set it off .
Diabetes in guinea-pigs especially older ones , can lead t weight loss .
Does she have a favourite food you can tempt her with? Most will kill for parsley . Or dandelions (wash and dry well , make sure they're not chilled)
It's a huge worry when they stop eating . You might need to get some Critical Care or similar (Fine Grind , mix up sloppy and use a small syringe or the tip of a teaspooon. Just behind the teeth, make sure she chews the spoon/syringe so she swallows , it mustn't go too quickly .
If you haven't got it you can mash pellets in warm water as an emergency .
I'd also add Vit C and Probitic Powder to the veg for an ill guinea-pig .
If she has a cagemate make sure she's not being bullied away from the food .
If you weigh her regularly then you can track her weight . Anything between 2-4 oz loss in an adult guinea-pig really needs to be checked out .
Good Luck .