Why is my mom acting like this?

A lot of times, my mom will say something really hurtful to me in a -- in her words -- "joking way", like when she says that I'm probably doing badly in history because I'm dumb, or that I'm getting fat around the waist, or other things that are offensive to me. Then, I'll tell her that it offends me, and I'll rant on about how she's being really offensive. Then, she says, "Just stop. I don't want to be attacked by my kids today." Then I'll inform her that if she says something mean or offensive to me, that me being angry with her is inevitable. She doesn't understand that actions have consequences. Also, we used to have a really weird sense of humor. Like, I'll ask, "what can I have for dessert?" and she'll suggest fruit, and then, in our way of joking with each other, say, "Yeah, like I *really* want fruit for dessert." Well, that happened today, and after I said that, she sighed, and said, "Honey, *please* just stop being mean to me!" We just got back from vacation in Mexico, where the entire time she was hounding me and my older sister, and her boyfriend, and telling us to go get something, or to do something while she lounged around at the beach, and pestering us to do things we wanted to, and just getting on our nerves, and we told her we didn't like it when she was doing this. She also said she didn't like it that we weren't appreciative to her about bringing us to Mexico, when every day --- literally, every day, maybe even every hour --- we were thanking her. Rrrrrgh! It's so aggravating! Why is she acting like this? I can't talk to her, because she'll immediately begin saying that she doesn't want to be attacked, when we are just speaking our feelings. What should I do?