It's nearly impossible for us to diagnose anything online.. so I'd suggest you take your snake to the vet. If possible bring a fresh fecal sample (might be difficult since the snake hasn't eaten recently) for the vet to check for parasites with. You may want to treat your snake for worms anyway (if you can't get a fecal sample) as a parasite infestation can cause lack of appetite and weight loss.

If you don't have a reptile vet, you can find one in your area with this website:
http://www.arav.org/Directory.htm