If you're trying to say that this is your first period in six months, then the main two causes could be a hormone imbalance which means you weren't actually going through the full menstrual cycle (in terms of shedding the lining of you uterus) which would be due to not having enough hormones to stimulate your body TO actually shed the lining; this can signal a problem with your pituitary gland. The other reason could be that you have lost a significant amount of weight and you have recently gained some weight, without fat on your body, you cannot produce the hormones needed to power the menstrual cycle. Any other number of reasons could be the issue ie. stress can be a factor. If this period continues to be irregular I suggest seeing a health professional.