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.