Help please IM NERVOUS?

okk so back like last year i went on some sort of diet and lost about 15 pounds but then i started back to eating whatever i wanted but i dont seem to be gaining any weight back and my sister said that it could be diabetes because her friend was diagnosed with it and she lost 30 pounds and never gained weight just kept losing and im nervous...i know this may sound stupid but also can eating a lot of sugar cause it and could some way by dieting could i have caused my matabolism to become faster? please helpppp!?