My mom keeps saying i'm fat?

You're not fat. At all. You are just fine, in fact I'm sure you could afford to gain a little weight. Let her know that what she's saying is hurting you, although honestly, it kind of sounds like that's something she wouldn't care about too much. Maybe find somebody you can talk to (a trusted adult/friend, school counselor?) about the way she's making you feel if she won't listen. As for the bra, you may have to either borrow one from a friend or buy one yourself.