Is it okay for me to spend my money?

The answer to this should be obvious but ill tell you anyway seeing as you are struggling. The money IS YOUR MONEY therefor you have a right to do whatever the hell you want with it. If your mum asks you for money again ask her what she needs it for if she refuses to tell you then refuse to give it to her. She is your mother she should be able to support herself and not scrounge of her children. As for you saving it up that's not a bad idea could come in useful if there is anything expensive you would want or need like a car or computer etc.