Underweight

ju

So last year I found out that I have too much sugar in my blood and I stopped eating sugar and fat for a month to see any changes... I always weighted the same since I stopped growing and now I lost some. Ok. Later on I was always hungry and ate more than usually but lost even more weight! I ate sugar and fat then just to see if I gain some weight. Nope. So I went from normal to underwaight in like 3 months and I don't know why? Can something else beside sugar be wrong? Did somebody lose weight while eating more? Is it possible that I can't get pregnant if too skinny but still have period?