Your mum us a single, non working parent. So, she lost a certain amount of money when you left education and started "work", which an apprenticeship is considered to be.
It's all well and good for those in the very privileged position to say shouldn't be charging you but the money to keep you has to come from somewhere and as an adult who is earning, that really should be you.
She will not have lost child benefit, as she will not have got any previously for your youngest sibling. She will now that she's not getting it for you.
She will have lost ££333.33 in universal credit..... although actually maybe not, same situation as child benefit.
Anyway, she may have lost maintenance money.
Welcome to the real world though. Adults, which is what you are, pay their way in life.
I myself would love £100 a month to do what I like with,never mind £100 a week.
No, you should not pay more, but you shouldn't pay less either.