The reasonable thing and go "You okay?" And then help them up. If it was anything more than that on their part, I think that'd be rather unsettling. They should've just struck up a conversation instead, or just asked me to move, etc.
What would you do if you had an obnoxious pimple that you tried to pop over and over that never seemed to form or go away? Keep trying, leave it alone, medicate it, have it checked out, etc?