I would have told her that she is going to regret what she has just done then
then sit down and continue my time at Macca's and wait for her to leave then follow her and ask her WTH her problem was and blah blah chances are she will talk with her fist and I will have to defend myself.
IF it was DH she would be picking herself up of the floor. The only time he hits a woman is if they purposely hurt his children.
You reacted in a very polite civilized way.