There's nothing wrong with using a low carb diet while on pred, although the pred munchies may make it difficult to stick with it. Light-headedness and hunger are normal when transitioning to a low carb diet. Try eating more frequent, smaller meals... six small meals/snacks instead of three large ones. You may also want to increase your intake of healthy fats - Udo's, flax, fish, olive, nut butters - to compensate for the low carbs and help keep your calorie count up.
Eating a grain-free diet is very helpful for healing UC, in my experience. Check out the Specific Carbohydrate Diet, if you haven't already.
Diagnosed with ulcerative colitis spring 1999
Maintenance dose sulfasalazine
Probiotics, l-glutamine and fish oil caps