Photoshop brushes now all "custom" mixing brushes

Hey all, 

I feel like a real idiot for not figuring this out. Probably user error. I've always used photoshop for editing, but I've recently begun painting in photoshop as I'm working on a webcomic. I rely heavily on the ol' classic "hard round" brush that comes with Photoshop. 

I used to be able to color a layer, and then using the hard round brush on the same layer, color right on top of that previous color section, and the brush maintains its hardness and roundness, like any marker or pen would. 

However out of completely nowhere, it's now a "custom" brush, and instead of creating a nice thick line on top of a previous color section, it turns into a blurry muck. So it looks like it has become a mixing brush or something. I definitely didn't mean to do this, and I can't figure out how to turn it back into the default hard round brush I know and love. 

All my brushes seem to be "custom" now instead of their default modes of operation. Can anyone explain what I did to deserve this? I've had no luck identifying this problem in previous forum posts or anywhere online and I'm sure it's an easy fix. I've tried restarting photoshop and no luck there.