Maggots. I had a hamster when I was 6 and it died, but I forgot to throw the hamster food out. I woke up one day to find them crawling on my ceiling. I felt really ill for the next few days, and the thought hit me that I probably also accidentally ate one because they fell off the ceiling easily. Also not helped by a very, very NSFW fic I read about a year ago involving maggots.
It's weird because just thinking about it just makes me feel a little ill but nothing else, but actually seeing them makes me throw up more times than not. Bonus points if I come across them in real life, as I will vomit on your favourite jeans.

I also get panic attacks when I go to see children's pantomimes, and I'm not sure why. Not performing in children's pantomimes, or going to see something like Shakespeare or Les Mis or something like that, but just going to see children's pantomimes. This I don't understand.