Width animation in After Effects

I already revealed my logo with a mask. The mask is with a line.

So as the line (it's just a thin rectangle) goes to the right, it reveals the text.

Next I want the line to grow thicker into a box.

Except after I add the keyframe, I change the width of the line and the Rectangle goes all the way to the left off the screen. Or I try to change the width and it doesn't change it for that key frame but throught out the animation like with the mask.

I don't get how this works, How can I animate it so that it won't move so wildly? All I want is to animate the thin rectangle to grow wider to the right side. Help? Adobe after effects Creative Cloud I'm using.