W.S.
Well it sounds a little confusing, but it seems like she was upset that you took something (Pepsi) without asking--now I know you thought the dad offered, but I'm just saying I wouldn't worry about taking a drink if she is offering it. I would probably apologize to her, saying "I'm sorry about the Pepsi, I thought (dad) said to get a drink and I thought I was being thoughtful by taking the one out of the freezer" or something. see what she says. Who keeps soda in the freezer? It will explode or be flat, I thought! Maybe she likes it super cold and had timed it just right to drink it when she got home?
I would also ask the dad what he actually said, when you thought he offered a drink, just to jog his memory, and maybe he'll remember that he did offer.
Do you normally walk in the door without knocking? Why did she make it a point to ask you to knock? If you do normally skip the knocking, I would be sure to start to knock first. It would bother me if my kid's fiancee walked in without knocking, I think. Perhaps your family had a different idea of privacy or courtesy than his family does, and you just need to learn their "rules?" It's not that she doesn't like you, but maybe she feels a little disrespected in these little ways?
You could SWEETLY ask her if there are things you do that bother her. I emphasize sweetly! Don't make it a confrontation or anything, she doesn't sound like she likes direct confrontation. Like after you apologize and explain about the Pepsi, you could say "please let me know if there is anything I do that is annoying or upsetting. I want to learn to fit in with your family" or something like that. Even if she denies anything, just the fact that you asked shows that you care about her feelings, and then the ball is in her court to tell you about it.
I would also tell your fiance that if his mom complains to him, he should tell her he doesn't want to hear it and that she should go directly to you instead, because it is not his job to relay her concerns. He should tell her that you are a reasonable person (assuming that you are!) and that you would want to know if she was bothered by something.
Did any of that ring a bell or help?
Blessings,
Lynn