Hello and welcome to the forum. Your dog's diet sounds OK. You say that she has no health problems - have you had her examined by your vet? I see she has had three litters - presumably one per year since she came into maturity at two years of age? As she has had a normal season since the last litter I gather that she whelped between three and six months ago? Without doubt pregnancy and nursing pups can take it out of a bitch and it can take six months or longer to fully get back to health and fitness. Have you finished breeding with her now? Based on what you have said, it would be best to retire her and think about having her speyed in three months time. As the other members have said, if you haven't done so already, pop along to your vet and have her examined. It could be any one of a number of things but something doesn't sound right and from your description of her diet it doesn't sound as if it related to that.