As a qualified (although not practicing) dietitian, I'd pretty much agree with hilldodger's statement. I've met quite a few nutritionists who really know their stuff. Unfortunately, there're also plenty who don't.
As for food allergies/intolerances , it's very unlikely (although not impossible). As most people have suggested, perhaps start by being really honest with yourself about the quaity of your existing diet and address any imbalance that may present itself. Reflecting on the balance of your overall lifestyle is a good thing for us all to do now and again, but particularly if you're not happy. Basic stuff, really.
As for dietician/dietitian, dietician with a c is an American spelling, and most dietitians get quite irritated by it (in my limited experience).
Hope you feel better soon.