"Ma sei una ragazza."
Perché is why, you probably meant però. Ma is close to but because of its usage as conjunction, but it's also used in exclamations of surprise: "ma no" (kind of "no way, that can't be!"), "ma come" (kind of "how is that possible"), "ma davvero" (kind of "are you for real?"). So, at times you need to get a little creative. Here I'd say it's simply but though.