Appendicitis. I described it as feeling like a Chestburster from Alien was gestating inside me and ready to chew its way out of me at any second. I needed near-lethal amounts of painkillers while awaiting surgery to not feel like I was about to die.
For comparison, I broke my foot last year and it still wasn't anywhere near as painful as appendicitis.
As a fellow ace, I get your frustrations, I'm sorry you have to deal with this.