So at first I though I just messed up a mask layer. Now, I thing there may be a hard or soft limit on how many layers and keyframes I can have.
I opened a new layer to add a glow effect to one of my animations. My new layer is unlocked, They are blank keyframes i'm drawing on. It's not even a mask layer, but a normal layer. And when I try to draw, the blank frame just... doesn't take it, even though it's selected and not even locked. And I've looked at my other layers, the brush stroke doesn't go to any of them. I even looked at my history, and I se no brushtroke in my reent history.