Bedding is the first question -
cedar is the worst.. if it's cedar or a mix of cedar or you even think it could be, dump it all and use newspaper until you can get something else.
Pine is used commonly, but alot of people have allergies to pine (myself included) and some as well. Even if many use it and swear it's OK.. it's not too many steps down from cedar imho as far as the amount of phenols it releases (smell from the oils) and I would not recommend it either really. Aspen is the best choice. Some use carefresh , it's safe (for cavies) I just found the odor a little much and it's expensive.
Oranges are good.. Kale as well, has alot of vit c in it, and you can use human grade liquid or disolve, tablet, vit c in water. But first and foremost, if seemingly everything else is covered and there may be a RI starting, I'd talk to a vet. They can come on fast and hard in cavies, but usually a vet can give some IV c, maybe some dex, and antibiotics for a quick recovery before it gest worse.
ON a side note.. dusty hay can also do it.. but I'd rule out RI first. Good luck!