I've been following this thread but haven't posted because I wasn't sure if my telling my story would add to the thread at all, but then something else happened to me that caused me to think more about this and I wanted to reply.
First, you were not rude to cancel. I think you were wise to do so. You were being put in a position where you were invited to come over and stay hungry while watching other people eat. And when you didn't eat, you'd either look like a jerk for snubbing your mom's food or like a liar for saying you're allergic to things while your mom could say that she didn't know you had those food issues. Even if everyone else knows about your food allergies, you'd still sit there sad and dejected while watching others eat. Or else you'd have to bring your own food and then look like a special snowflake for doing that. And it would be an insult to you, in front of others, that your own mother doesn't care enough to cook something you can eat.
I can relate to this situation. And I agree with the PPs that say that there's more going on here.
I'm my parents' only child. But my mother always claimed not to remember that I am allergic to nuts. Now, my nut allergy has never landed me in the ER. I just have my throat close up and my mouth itches, but I can survive a few bites from food that has nuts in it, and have done so on many occasions when I didn't know there would be nuts. But I don't eat a whole piece of whatever. My mother knows this because my allergy is not new.
I had a strained relationship with my parents and rarely saw them. Then, when I was pregnant, my mother wanted to be grandmother of the year and insisted upon throwing me a baby shower. It was going to be a shower for her friends and family only. After I resisted, and after a lot of convincing on her part, I figured, for her sake, because she was excited, I would accept the offer.
There is more to the story of our poor relationship. Then, when dessert came, she served a carrot cake that she made from scratch. I generally prefer chocolate cake. She announced "I made carrot cake because Danika loves carrot cake." I like it, not love it. But I figured there was no point in correcting her publicly and saying I don't love it... and then it had nuts in it. She baked it from scratch. There was no need for her to add nuts to it.
So I didn't even get to eat a piece of cake at a party that was allegedly for me. And when her friends asked me in a shocked tone why I wasn't having any cake, I replied "I'm allergic to nuts." And they looked at me incredulously and said "why would your mom put nuts in the cake then?" I replied "I don't know."
When I mentioned to my mom for the 1,000,000th time that I'm allergic to nuts, because I've told her at many other events, she said what she always says "I forgot." She has no food allergies. I think she thinks I'm making it up. Although, she has seen me have more severe reactions to other foods to which I am allergic (like Florida Fuerte Avocados). I can't fake swollen lips and a swollen tongue, so it's not just my imagination.
I was never sure why she put nuts in the cake. Maybe she wanted to test to see if I had a bad reaction, to call my bluff because she thinks I made the allergy up. She thinks all allergies are fake, not just mine, all allergies that any humans have. But there's more in our history. She always makes food, whines if I don't put enough on my plate and eat enough, and then calls me fat. So maybe she figured I was getting fat in my pregnancy and was "helping" me by not offering a dessert I could eat. At a party that I only attended to make her happy, by the way, because I didn't want that baby shower.
My mother is emotionally and verbally abusive in other ways. This is a small example. I wasn't going to post all of this here because it's a lot of drama and a tangent.
However, I got an email from an acquaintance yesterday and that put it in perspective for me so I wanted to post that tidbit.
We have a group of neighbors (ETA: same general neighborhood, but not living on the same street) who get together from time to time. I've met this acquaintance twice, never at her house or mine, but just at gatherings around town. I must have mentioned at some point that my doctor had me on a gluten-free diet, but I don't remember saying that. She's having an event at her house next weekend and it's potlock. I emailed her to get her address and she replied that she remembered that I had been on a gluten-free diet and wondered if I still were because she would modify what she planned to make.
This acquaintance, basically a stranger, remembered a diet that I was on for a limited time and was offering to help me! My own mother could not remember a life-long allergy of her only child, and when she did "accidentally" put nuts in something and was corrected by me and her friends, didn't even apologize.
My acquaintance/neighbor is more thoughtful and considerate than your mother and mine. Which explains why my mother has now been cut off completely for the past two years. Obviously, there's more to the story between me and my mother. I suspect there is with you too.
I say, good for you for protecting yourself from her manipulations. You were not rude. And even based on just this incident, I don't think she's very caring. Don't go. Be proud of yourself for protecting yourself and standing up for yourself.