This sounds to me like a blood glucose crash, which hit me especially hard back in the day when "we'd all go out to eat" and have a larger lunch than usual. Typically, such a lunch incorporates a lot of carbs: buns, chips, fries, rice, and more (even that yummy fresh-baked bread). Highly processed carbs - anything containing things like "modified food starch" - tend to hit harder, as they're essentially sugars all chained together that become unbound the moment they start digesting and hit the bloodstream right away.

The problem is typically addressed by changing to some version of a low carb diet, which includes things like keto and carnivore diets, though those are more specialized. You don't have to entirely eliminate carbs, because not only is that impossible, your liver will generate them if your carb intake is very low. The point is to be mindful of what you are eating and especially avoid processed carbs, which isn't just donuts and cakes, but pretty much any snack you can get out of a vending machine or otherwise comes in a wrapper. Even whole wheat bread can cause a sugar spike/crash if you consume too much.

I've found that the couple of slices of bread on a sandwich typically don't cause the crash if there's no other carbs in a meal, but these days I even avoid that. I'll instead make a bacon double cheeseburger with no bun, for example. It sounds unhealthy, but except saturated fats, it's a very nutritious meal with loads of B vitamins and vital proteins that health nuts pay big bucks to consume as a powder in a smoothie. And there's evidence to indicate that saturated fats aren't as bad as they were portrayed under the old 1990s food pyramid.