Jaybay has given you some excellent advice. Please take her to a vet and get her checked out ASAP. What you have to keep in mind with animals is that even though they are domesticated and live with us as family members, many of their wild instincts are still intact, and they are expert at hiding health problems because in the wild, to appear ill automatically makes them prey to other animals. They are SO good at hiding it, in fact, that many times by the time they begin to actually show signs of illness, the problem is quite well advanced and much more difficult to treat. She needs to have a thorough going-over, probably with a full blood panel to see what's up. Please keep us posted.
Ghilly
Please take your dog to a vet. Lack of appetite and weight loss means something is wrong. Hopefully whatever it is can be easily treated and have her healthy very soon. There is no point in trying to figure out the problem before you go to the vet - that's the vet's job. The sooner you get the problem properly diagnosed and treated, the sooner she will get well. You don't want to wait until it becomes a complicated, expensive or even fatal condition before seeking medical help.