vomiting, sinusitis, laryngitis, etc. are not signs of health. being sick makes you tired since your body is trying to repair itself. you should see a doctor. that being said, these are not caused by improving your diet, but something infectious.
if you're going low-carb at the same time (are you?) that may add to your lethargy, as your body switches from sugar-burning to fat-burning. what do you typically eat each day?
As I ate the oysters with their strong taste of the sea and their faint metallic taste that the cold white wine washed away, leaving only the sea taste and the succulent texture, and as I drank their cold liquid from each shell and washed it down with the crisp taste of the wine, I lost the empty feeling and began to be happy and to make plans.
– Ernest Hemingway