Why do masks sometimes fill in white and sometimes black when I move a selected portion of the mask?

For reference, I'm using PS to draw by making solid color layers and applying masks, usually by painting 100% white with a 100% hardness round brush. I often want to revise these masks after making them. One way I do this is by selecting a portion of the mask with the lasso usually, and then transforming it somehow. This brings me to my problem. If I translate the selection without scaling it, sometimes it automatically fills in a part of the mask white (resulting in visible color that I don't want displayed) that is the exact shape and location of the original selection. I'd like that portion of the mask to be filled in black every single time. Anybody know why this is happening or how I can get PS to fill that portion in black every time? Thanks in advance!