What Causes Irregular Menstrual Cycle?

Hi, I am 25 year old and unmarried, weigh 52kgs. I have irregular periods since last 4 years. An year ago there were no periods for 4 months after which I consulted a gynecologist. She prescribed krimson 35 for 3 months followed by Deviry for 2 months after hormonal tests. LH was high, all others were fine. Scan dint show any cysts . Cycles were proper when I was taking medicines though the flow was less. Its been 3 months I stopped medicines. Last two months periods were proper. But I missed periods this time. I struggle a bit with falling asleep at the right time. Also there is a bit of stress and work load. I try to go for brisk walk for 45 min , twice a week. What can I do to get my cycles right? Its been more than 20 days since due date. Please suggest. What are the factors that cause abnormal periods? Thanks and Regards