Is something wrong with me ?

I need advice is something wrong with me?

At my job I was hired, because a lady there before me went on maternity leave. My manager says that when she comes back from maternity leave with myself or her will have to go part time, Which is unfair. She mentions it a lot as if she wants her to come back on full time and I automatically just go part time? The problem with this is that there are similar issues that keeps popping up to where things are unfair on my end. Like the new employee that was hired is not required to work on hard days and if she has no other choice but to the manger will offer to buy her lunch for her time. I noticed I have to work every week end and the others get to be off. And when I do work the manager will examine everything I do closely for errors. I’m confused as to why I’m being treated this way. Is it because I come off as dumb or slow?