My 25 year old daughter's menstrual cycle is a little messed up. About 5 years ago she suddenly missed a period for no reason. Then went back to normal, only for her cycle to get longer and sometimes shorter. Then this year she missed another period and sometimes bled for 2 weeks. Blood Tests and ultrasound did not show any abnormality. What could be causing it? She is a very anxious young woman, has a very demanding job..could all this really be down to stress?