Depressed For No Reason

Hi, I'm a 14-year-old girl with a boring life. I don't have many friends and I'm an outsider, but that's not my problem. What started this year is not going well for me at school and with some friendships. It went somewhere, but about a month ago I was depressed for no reason for the 1st time, I wanted to cry and I was very sad and I thought it must be because of that. Yes but no. At the beginning of the month, I realized that I was depressed for no reason. At school I sit happy and suddenly I get sad for no reason and I don't show it. Sometimes I'm happy at school and when I leave school I feel sad as if I'm missing something, and I don't like school and my classmates at all, especially because of them, otherwise my teachers like me. At home when I'm and I feel depressed and I play sad songs and videos and cry. It may seem too much to you, but my success in school fell because of this causeless depression, not much but again I lost success in some subjects by one unit down. In English I had only 6s, and now I earn only 5, in biology I had only 5 and now only 4s and when I think only about these subjects. Help me, I used to get depressed for no reason, but not as often as I do now. Once a month I was depressed a month, and now once a day at least. I'm depressed and then it passes, and then depression again. Okay, I don't have any friends but I'm used to it and I like being alone, so don't give me advice to look for friends. I am a person who loves loneliness and loves to be invisible. I also have family problems, but I don't care about them anymore, because I have my mother, grandmother and some cousins ​​and I'm fine with that as well. I have no problems for myself, except for this causeless depression.