I recently had a problem with Photoshop I haven't had before. When I draw with the Pencil Tool on, or next to, an existing pixel, it makes the color the background color. This started yesterday.
Today, when I started working, the same problem still occurs, and now my pixels are all semi-transparent. I don't recall clicking any buttons that I don't usually, and haven't done anything new with the software.
I added a Youtube link to my problem (its awful quality, my computer is trash) but you can see that my pixels are semi-transparent, where you can see the color and the background squares. You can also see how when I draw over lines, the cyan background color starts drawing instead of the reddish color thats selected as the foreground color.