Why do i have mood swings sometimes?

Though I didn't know it at the time, I would say the first sign(s) of my mental issues were in 11th grade, I was very annoying and sometimes I was hyper and it was like I couldn't sit still (but I don't think I had/have add/adhd as I could focus really good in school), and sometimes I would cause myself pain by hitting my arm against lockers (which was in a way that it was annoying to many), and I would also sit in a way that one side of my chest was on the desk; and it hurt but I loved it. Then in 12th grade I had become depressed and for a while I just couldn't be happy; this is when I started cutting myself. after a while in addition to the depression, anxiety, and cutting I had mood swings from depressed to manic; or at least I think they were manic spells, usually I would have a lot energy and want to do so much anywhere from for a few minutes to a few days, and eventually id go back to depressed and would struggle to get out of bed (I think I may be bipolar sometimes, but ive been getting help and Idk what my diagnosis is). I also wouldn't hesitate to get physically when middle school kids when I was in high school (they used the same busses). I think I still have some degree of mood swings, though I feel stable now. So why do I have mood swings?