For me, post meal sleepiness often equates to out-of-range blood glucose levels. I get tired and full of yawns when my blood glucose is low or high, especially if it's climbing quickly.
If you share these symptoms with me, it will be easy to check and see it it may be causing your drowsiness. Eat a meal and then do a fingerstick check at mealtime and every 30 minutes thereafter out to four hours post-meal. Do this for at least three meals at different times of day before you draw any conclusions.
On rare occasion, I will consume some extravagant treat, like a chocolate milkshake. There's no way I can effectively dose insulin to stay up with that radical of a post meal food spike, but I try nonetheless. As my blood sugar rises and accelerates after my decadent treat, I usually feel overwhelmingly drowsy and all I want to do is climb in bed and close my eyes. This despite having adequate sleep overall.
It's just a thought. I wish you the best in solving this puzzle. Your curiosity is healthy; I only wish doctors would strongly follow-up on these kind of symptom reports. I guess there are not enough hours in the day for them.