Is my co-worker allowed to do this to me?

We were what you might call friends. I saw a few red flags that our friendship was over... but the last time I spoke with her, she & another woman (who she never liked before) cornered me in the breakroom & rudely confronted me about my upcoming shift change. They accused me of causing them to lose the new hours they were trying to get approved, even though many people didn't want it & all would have to agree. That was my last day on that shift. I no longer see her during my hours at work. I made no further attempts to contact her because of her betrayal. I make nozzles that she puts in cases of product. I make mistakes but less than her. She actually went home & printed up a worksheet in her free time to fill out at work about every mistake I make. She has been handing them in with her required worksheets each day. My boss has told me not to worry but yet the sheets are being kept in a binder. It has been 5 mos. and the other 2 ppl who do her job never do it, just her.