problems with sculpting a head

Im watching a tutorial from blendercookie, on how to model a human head just using the starting cube, i had no trouble sculpting the mesh for the head, but i erased some of my grease pencil strokes, and after that it stopped working, whenever i would use the grab tool for sculpting it seemed to grab things from inside the mesh, and create bumps all over the surface of it, it wouldn’t just sculpt like it was supposed to, and whenever i go into edit mode my mesh would turn into a cube, i tried going into edit mode several times before this, and it was fine.
Running Blender 2.61
on windows vista 32-bit