LaTeX One \Fill moves the other to the side

So I need to draw a flag with red background and an X in the center. I decided to do this using the \fill command three times in order to make one big red rectangle in the BG and two rectangles to form the shape of an X. The problem is that after I have made the background and one of the rectangles for the X, the second one moves everything to the side.

How can I move the black rectangle on top of the red one so that it is in the center without moving all of it to the left or right?

P.S: Looks like this without the black rectangle

------ Code Down Below ------
\begin{center}
\section{Flagga}
\begin{tikzpicture}
\fill[red] rectangle (12, 8);
\fill[white, rotate=31] (-3,-2.5)+(1,2) rectangle (15,1);
\fill[black, rotate=141] rectangle (10,1);
\end{tikzpicture}
\end{center}

• Welcome to TeX.SX! Please make your code compilable. – TeXnician Nov 24 '17 at 9:32

Like this?

\documentclass[tikz,border=2mm]{standalone}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}

\clip rectangle (12,8);
\fill[red] rectangle (12,8);
\draw[line width=8mm, white] (0,0)--(12,8);
\draw[line width=8mm] (0,8)--(12,0);
\end{tikzpicture}
\end{document}