Weird Gray Box Thing?

I'm working on a final project for school, and everytime I try drawing on a specific point of my artboard, this weird gray box shows up instead of the actual draw lines, and I have to constantly undo all my work to get rid of it. I'm not sure if it's because I'm using too many layers or what, but I've had a Photoshop project with more than 20 layers and it didn't do this before.

Any reason as to why this is? It's getting very frustrating and I'm unable to make progress because of this.