I know it may be hard to hear, but if you spend all your time on WCRPG (like you've said), she may just be trying to help you. Your parents want what's best for you, and if you're not taking things in moderation, it's not the best. It can be hard to see that, especially when you are young, but this site is not the do-all-end-all.
It's not healthy to sit all day, whether you eat right or not. Between your laptop and school, it's bad on your muscles, you bones, your organs; a lot of things. There's also the mental side of things; you say you are stressed out. A website should never be impacting you like that, ever. School in understandable, but WCRPG is purely recreational. By putting yourself under she emotions consistently, you are at risk for depression and other mental illnesses.
I'm not saying quit the site; I don't know you, I don't know your mom. However, I really encourage you to do other things. A lot of people get sucked into things and have to break out, like with video games. There is nothing wrong with playing games, even daily. However, you can get addicted to these things, and it is a very real thing. If your mother sees that you are doing different things everyday, I'm willing to bet she will give you back your laptop. Go outside, join a club, whatever. Don't let this, or anything else, consume you. :)