libgdx - Why does resizing my game window move and distort my rendering?

I'm starting to make a game, and the first screen I've made is a very simple loading screen with some centered text.

On starting it up, the text is nicely centered:

Then I resize the window, and I get:

Why is it stretched? And why has it moved? Looking at my code it looks like everything should be perfectly centered:

public void render(float delta) {
        Gdx.gl.glClearColor(0, 0, 0.2f, 1);

        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        if (Assets.manager.update()) {
            // Eventually change to a main menu or something
        }

        String text = "Loading " + Math.round(Assets.manager.getProgress()) + "%";

        BitmapFont.TextBounds bounds = fnt.getBounds(text);


        batch.begin();
        fnt.draw(batch, text, (Gdx.graphics.getWidth()/2)-(bounds.width/2), (Gdx.graphics.getHeight()/2)-(bounds.height/2));
        batch.end();
    }