I take some medicine and vitamins for appetite maybe this is the cause of menstrual late period?

Dexamethasone: I do not know of any vitamins that will affect your menstrual cycle but see that you are taking dexamethasone. This is a powerful medicine with many side effects that is known to disrupt your menstrual cycle (as well as other hormones). I would use this medicine with great caution and discuss other options with your doctor. Consider acupuncture and herbal alternatives.