Color problem- Help! [Fixed]

[choklitchan]

I have Photoshop Elements, and today my brush started doing something weird. If I brush in a color, and then try to paint over it with the same color, in the same layer, the result looks like this.

As a result, I have to create a whole new layer for another stroke of the same color.
What can I do to get my settings to the way they were before?

Edit:
This was nothing major; nevermind...
Also; I tried getting my Levels to work by using the black dropper on dark pencil lines, and white dropper on the background, and nothing happens, even though the graph looks a little changed. What's wrong?

Thanks for reading this.

[Zumi]

Looks to me like the "Mode" for the brush might be set to Exclusion.

[choklitchan]

That's exactly it! Thank you so much.