I’m trying to think about something that might have caused me emotional discomfort between yesterday and today,
No, doing this is not going to be helpful to you at all, because it is not that precise or logical.
This is why we use pen and paper. The purpose of the writing exercises is to quickly write down ANYTHING that is
on your mind, no matter how random it might be, and no matter when it occurred in real life, and
especially if you think that you can skip something as "not important". The faster you scribble down your thoughts, the better, so that you don't have time to edit or "fix" them.
The things you need to uncover and examine are the thoughts that your brain felt that it needs to repress (and replace with pain) and yes, it apparently found a new reason to do this sometime between yesterday and today. HOWEVER, the actual incident or interaction could be something which happened last week, last month, or a long time ago - which was recently triggered into memory, perhaps in the last week or so for some reason (sometimes by something we read or see or hear, or a personal interaction).
It IS random, which I understand is terrifying for you, as you have acknowledged that you have an extreme attachment to perfectionism (one might say obsessively so).
You are terrified that things might be out of your control - which is logically irrational. If you will stop being so terrified and apply your logic, you might be able to accept the truth that you are perfectly safe, and that uncertainty and imperfection are not going to physically hurt you.
How can I rule out a physical cause?
If the doctors have already said that they can't explain your pain, it is quite illogical to think that something could suddenly now be responsible. If you want to go back for another examination, that is your choice, but it will more likely be a waste of time and energy that would be better spent by reducing your anxiety and perfectionism! There are many Success Stories here where other people describe the futility of their multiple doctor visits because they were so afraid of "injuries" that did not exist. When they properly reduced their anxiety and addressed their repressed emotions with courage and acceptance, they stopped looking for physical "causes" and they recovered.