#eraser effect

There are a few possibilities, assuming you have the Brush tool selected, not the Eraser tool (it doesn’t matter what is selected in the brush list, check the toolbar).

However, the most likely cause is that you have the “transparency colour chip” selected, which makes any brush into an eraser (this can be very useful for some artistic effects).

If the Z key on your keyboard is stuck (or you have some sort of keyboard macro program that presses the Z key), that will have the same effect (I once got something similar when the corner of a book was resting on the keyboard).