Why am I losing so much hair?

Ay

For months now my hair has been falling in small clumps every time I finish washing it. Even just brushing my hair lightly with my hand will cause 2-5 strands to fall out. As of right now I have around 1/3 of my original volume of hair from last year. My mom said it was because my hair is too long so she cut it, and she also gave me a bunch of these vitamin supplements to help more grow which I don’t think it’s working. Is there another reason why I might be losing hair?