Brush tool randomly filling in blank interiors (I can't explain it better)

I'm having a huge problem where I will draw a line with the brush tool, then (not necessarily even close to the target zone) another spot will randomly fill itself out.

Usually all I would do is just CTRL-Z but flash doesn't have any memory of making this giant mistake that can cost hours of work, so it can't be undone unless I go back and erase it all and restart the part that became filled. Nothing strange happened before the glitch and flash just carries on, as I said, without any recollection of anything.

I don't know if I can explain it better than that. there's a bunch of brush strokes, then it's filled with color that I can't undo.

Please help me.