Yeah I suggest just expressing your discomfort to your friends. Say you're trying to go vegan/vegetarian, or even just cutting back on it if you don't want to commit to a label. Echoing what others have said, people tend to be accommodating in my experience. If you're ordering food, most of the time that means just looking at a menu beforehand and picking a different place in the uncommon case where there's nothing available. And in situations where it's hard to accommodate, you can have snacks ready or eat before/after a gathering.

I wonder if it's the iron and/or the comparatively large sudden dose of peptides overwhelming my proteases

Actually the iron would make a lot of sense since I can't even take plant iron supplements on an empty stomach, and seem to be especially sensitive to it

marinated grilled mushrooms sounds bomb.

Argh, I had protein bars in my bag and completely didn't even think of them!!! 🤦‍♀️

I was worried about being a pain in the ass too when I went vegan! Surprisingly, no one so far has been anything but kind and accommodating when I asked them to get me something else. Even my (usually) insane conservative parents were 100% on board, no questions asked. So maybe just try asking and you'll be surprised how little they mind!

A little trick I also like to use is to specifically request a plant based dish that the host already likes making. So they don't feel lost trying to find an unspecified alternative. For example, at the last grill party with my parents I asked my mom to make the marinated champignons that she loves making. She was really excited about it haha

Definitely consider bringing your own snacks atleast. It does wonders

psychological

You absolutely have a physical reaction, your gut microbiome is not used to animal based food.

(and the biochemical difference is nutrient availability)