I think Annieh's suggestions are good and she has a point about the sweetener. The craving for sweet things can actually represent or mask a craving for food. I'm thin too, and lost weight on the palaeo diet which I didn't need to lose because I was taking everything to heart on the blog and the forum without considering that actually I wasn't someone who needed to cut the carbs.
Given that you say your energy levels are no better, I'm wondering if, in fact, you are simply eating enough? If you are putting stress on your body by not providing it with enough fuel, could that be having an impact on your readings?
But look, I'm not a doctor and I don't know all your details. I just know that I eat LOADS MORE food on a primal diet than I did on a conventional one. And I still weigh less than before but have bags of energy and none of the minor health issues that previously plagued me.
Maybe someone can offer a more scientific insight that will help you. Best of luck on your journey.