False Erasing

[Former user]

For some reason sometimes when i click on a new color(using the pencil tool) and start to draw on the canvas, as soon as i left click, the area where my cursor is will be erased like as if i used the eraser tool. Also after i let go of the left click button it still continues erasing when i move my mouse. Lastly, the area erased is not "pixel perfect" as shown on the image attached, it can erase half of a pixel too. This happens randomly and will only go away for some time if i select the art with the selection tool and move it.