My fiance has always been very emotional and extremely easy to have her pissed at me. I used to think it was bad PMS, and she does have bad PMS but when I determined when she gets frustrated or angry at me I learned that the timing of her period has nothing to do with it. I'm very calm and have tons of patience for her and there were plenty of times I've had to basically cover for her after she makes someone feel bad or is rude enough to me in public that it affects others. I've come to accept that emotionally we are opposite.
My problem are the things she gets mad at me. I feel like they are mostly universal to relationships but I don't know. For example, today we are working together and boss is treating us to lunch. We ate light and split a burger w/ fries and a drink. I ask her "what soda do you want?" and she says, "oh you choose," so I pour Mt. Dew. When we sit down to eat she gets flustered and mad that I got Mt. Dew. "Do you want something else?" I ask and she says, "Oh no it's fine" in a very condescending and bitchy manner, refuses to drink it until I get another soda. Later her reasoning is along the lines of "well we don't even drink mt. dew."
I barely drink soda at all. I don't stock it in my fridge. I don't care what kind of soda I'm getting, it's all the same sugar to me, it could be pork sausage flavored for all I care. This sort of moment happens a lot with us and she makes me feel like I'm her bad child or something. What I got out of it was that in her mind I should know what kind of soda she wants without having to ask her, which is crazy since she's always telling me we need better communication. And when I do what she wants and act for myself she gets mad about it, and if I act while trying to get her opinion she gets mad in a different way. When she's mad at me she wants me to tell her the problems I have with her, and when I do that she gets even more mad. Is this normal, is she selfish, or am I just stupid?
My problem are the things she gets mad at me. I feel like they are mostly universal to relationships but I don't know. For example, today we are working together and boss is treating us to lunch. We ate light and split a burger w/ fries and a drink. I ask her "what soda do you want?" and she says, "oh you choose," so I pour Mt. Dew. When we sit down to eat she gets flustered and mad that I got Mt. Dew. "Do you want something else?" I ask and she says, "Oh no it's fine" in a very condescending and bitchy manner, refuses to drink it until I get another soda. Later her reasoning is along the lines of "well we don't even drink mt. dew."
I barely drink soda at all. I don't stock it in my fridge. I don't care what kind of soda I'm getting, it's all the same sugar to me, it could be pork sausage flavored for all I care. This sort of moment happens a lot with us and she makes me feel like I'm her bad child or something. What I got out of it was that in her mind I should know what kind of soda she wants without having to ask her, which is crazy since she's always telling me we need better communication. And when I do what she wants and act for myself she gets mad about it, and if I act while trying to get her opinion she gets mad in a different way. When she's mad at me she wants me to tell her the problems I have with her, and when I do that she gets even more mad. Is this normal, is she selfish, or am I just stupid?

She doesn't seem to want you to be yourself at all, not really. She says you need improved communication, but it seems more like she only wants you to understand how she expects you to think and act around her. She probably wants you to be constantly aware of her needs and expectations--and this awareness needs to be part of who you are, a.k.a. "being yourself". So maybe she wants you to be her version of you, not the You who you already are.
You could talk to her about how you feel and ask her why she treats you this way (has she lost respect for you?). You need to figure it out before you get married because it's one of those things which will only get worse with time. She cannot control you, you are not a child; she must she accept you as you are.
+1
So in that sense it's completely normal. Is it necessarily healthy? No. The only advice there really is to give is that you should try to have a calm talk with her about this. Although, you are risking that The Bitch shows up again. And in that case, the only options left are counselling or leaving.