The pattern goes: I feel BAD -> I wonder why -> My brain makes up a BS story why: e.g. "That bad moment from today", "Worried about that thing tomorrow."
When I dig into the actual details behind that state it's almost always triggered by one of three things.
a. Low SALT. Can be proven by taking a pinch of salt with water and seeing how every ounce of 'tired' disappears in 20 minutes.
b. Low Blood Sugar. Usually accompanied by cravings for food, and specifically carbs. Thoughts of eating a steak aren't as enticing as a bowl of rice. Can be proven by eating some carbs and watching as the "feeling bad" disappears...often returning later as...
c. High Blood Sugar induced Insulin Spike This is a lethargic feeling. Thoughts of doing a pushup, or thoughts of doing literally any work are vomit inducing. This can be proven by simply waiting 2-3 hours without eating and watching as 'feeling bad' entirely disappears.
In more technical words. A) is an electrolyte disbalance. B) is the fat-buring / ketosis transition and C) is the side-effects on Insulin on the brain.
I've an extensive experimenter and do many things on my body through freediving, fasting, bodybuilding. I can cause myself to 'feel bad' on command by any of these three things. And after hundreds of intentional and accidental 'feel bad's, I've realized it's a stupid simple pattern.