Hair loss ??

I am 16 turning 17 and I’ve lost a lot of hair over the last dew months . I can visibly see when I lift up my hair sparse parts at the roots and balding patches . Could someone let me know a possible explanation as to why this has happened , whether I shluld be worried and go to a doctor, and things I can do to help my hair grow back xx

Thank you , everything would be appreciated this subject makes me feel very teary c