Wow that sounds retarded. Sounds like your boss was not fully aware of what to do in the first place--because if she did, then she would have told you everything up front.
I know everybody defines the word "injured" in different ways. I believe that there is an "injury" and there is being "hurt." Being hurt is like having a paper cut on your finger. And being injured renders you unable to do normal day-to-day activities.
That is stupid--all you did was say you got dirt in your eye...and she said you were injured? I think that is total BS. I would request for a sit-down and have her explain all of this to you. Tell her you were NOT injured. You just got dirt in your eye...it wasn't like you cut yourself bad or had a heavy piece of equipment fall on you and break a bone.
Tell her you were not injured, had no supervisor on duty at the time of your shift, and that you didn't sign a form.