Mesh distortion problem while sculpting

Hello,

I am currently trying to do the beginners sculpting exercise on CGcookies and while trying to sculpt the mouth cavity I run into the following problem. While trying to push the mesh inwards it started creating those weird IDK what it even is.
The black mesh is only the mask brush so the mesh is ignored while sculpting
First time uploading the picture so I hope it wont be too small


What causes such a problem? Maybe i added too much smoothing detail too soon?
How can I fix/ avoid problem like this in the future?

Thank you for your help