Willow and Hearing

Yesterday I was about to open the truck door when Willow materialized.  She always does that.  Normally I open the truck door and let her jump in.  But this time was different.

I told her she could not go.

She instantly turned around and went down the deck stairs to the back of the house.  It surprised me.  Usually I think she doesn't understand what I say.

I think I heard her say "OK".  Or was it, "Whatever."?