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Mood swings with frequent low mood episodes can occur due to lots of causes. If there is variation in a menstrual cycle, it can be a premenstrual syndrome. If you have a predominantly low mood, it can be dysthymia which is a low grade chronic long-term depression. As you are living in a hostel, it is very obvious to lack proper family support. You should have adequate talk to your parents. I know it is tough to visit your home frequently as you are studying.
Try to share your thoughts with your friends. Make such friends who listen to you and take proper care of you. If still, you face the similar problem, I would suggest you visit a psychiatrist for appropriate assessment. Do not worry. I am sure that you will be fine with a proper friend and family support. You do not require antidepressants or mood stabilizer drugs.