Why is my boss being like this?

I have worked at my place of work for about 8 months. At first, my boss was firm yet fair. Recently, she has been treating me very unfairly, pointing out negatives rather than positives, constantly threatening me with warnings if I did the smallest thing wrong/not to her taste. I have not seen her act this way with anyone else apart from me. It is almost like she WANTS to make me leave as she has no reason to fire me. I am really getting fed up and it's getting to the point of dreading every shift. I only work there part time, I normally had 3 or 4 shifts a week but now it has came to 1 shift a week or even none. I don't know why I have suddenly been turned on but I can't stand the constant aggression that is put towards me! I am looking for new jobs and hope to be leaving there soon - do you think this is wrong? Would I be defeating myself and letting them win?