I had the same type of situation you are describing with weight loss, although I don't consider it “normal”….for me it turned out to be leaky gut with severe malabsorption. You can order tests for malabsorption yourself through enterolab. As far as the weight coming back: I lost over twenty pounds, most of that since going grain free. I have been grain free (also dairy, soy, and refined sugar free) for over a year and a half. I did not gain any weight after going grain free, but I know it's better for me in the long run. My doctor ran some tests and I ended up needing digestive enzymes, l-glutamine which is an amino acid to help heal leaky gut, and high dose probiotics. I also have many other food intolerances which contribute to inflammation and leaky gut. Since starting all this I've gained about five pounds, which I am delighted for, but I'd still like to gain at least five more to be at a healthy BMI.
I would recommend that you find a good functional medicince doctor in your area.