Hi young girl, you are in your teens when all your sex hormones are beginning to peak and this is the reason you have acne, slight weight gain, heavy periods due to early anovulatory cycles. Do not put on weight, get into some exercise schedule, your hormones will automatically fall in place.Visit your gynaecologist if your periods continue to be heavy, you may need to be further investigated if required, your craving for sweet foods could be normal but do report to your physician if you feel weak, have frequent urination etc to rule out early onset diabetes but you do not gain weight if that is the case.