Not eating, but sleeping a lot

I have a 12 month old Cavalier who just will not eat and she sleeps a lot, the vet says there is nothing wrong with her and when she is running round on her walks I would agree. But I really am worried because she eats little and dog food she won't touch we have tried just about every food on the market. She like to eat what we eat if she gets chance but I am against this I have never had a problem with getting a dog to eat before and this is worrying me, I had my doubts when I brought her and she had to be spayed when she was still very young for health reason. She gives the impression she had to fight to get any food as a puppy and I wonder if her behaviour could be psychological.
I know she misses our old dog because when she sees a German Shepherd she automatically goes up to them as if it was our old boy. I can't afford to get another dog but wonder if this also could be the problem as she ate fairly well before we lost him. The strange thing is though she has not lost weight through not eating. Do any of your readers have any ideas.
Mrs Rose Tyrrell, Coventry