"May I help you" reads like a nice phrase, but what if the same gentle phrase is said in a tone that says "what the f&*k are you doing here?"  Then what do you say?  
What if this is the third time this same person says it the same way to you?  For no other apparent reason than what seems to be the fact that she can't stand the sight of you, that you are anathema and the most logical conclusion is that she is a damn racist?
What do you say to that?