Will Irregular Periods Affect Normal Activities And Growth?

My daughter is going to be 15 yrs. this month end. Her periods started last year March 2010. She has irregular periods. She skipped last month period and has not started till now. She feels very tired and lethargic. Last month was very hectic in her school, lots of activities, this month she is preparing for her exams. She is not feeling energetic at all. Kindly advise me. Even the glow on her face , complexion and height has been affected. Kindly advise.