Not knowing more about you, what you have been through it is hard for me to say - and yes, I’ve experienced very similar swings. It could very well be that you have a hidden infection somewhere in your body, an infection that may not yet be causing any apparent symptoms other than in you Body Glucose Level [BGL].
Like you, my breakfasts and lunches stay the same most days and by experience in taking the same amounts I know the carb count and take insulin accordingly; dinnertime carb counting has been my challenge. If I make a significant mistake undercounting dinner carbs, we eat out 2 or 3 evenings a week, the repercussion of the undercount takes a while to correct. I have found that when I go “high”, such as 250 or higher, my usual correction doses based on my insulin sensitivity need to be doubled; but then need to watch for possible insulin stacking in days following.
You mention “exercised excessively”; keep in mind that aerobic exercising, moderate levels, tend to lower BGL while heave anaerobic, or strenuous, exercising tends to increase BGL. I have found years ago that accompanying my wife to a mall and “strolling” about for a few hours will cause my BGL to drop significantly while walking at a brisk pace, say 4+ mph, or medium fast biking will cause my BGL to stay level or increase. The point of change has taken me years to identify and each of us may be different.
Add to this, that three different endocrinologists have at different times in my LWD diagnosed me as “brittle”. With my Minimed pumps, I will get basal rates “perfect” by long-term fasting and then rum very steady BGLs for several days / weeks [I usually check BG 8 times each day] and suddenly everything goes haywire - completely unexplained.