I get the same thing. Not after breakfast. I'm fine then. I get it after dinner which I eat around 6pm, and then I eat a full balanced meal, of usually quite healthy foods (even though with some IBS related restrictions.)
Then I am not just "sluggish", I have a totally irresistible need to sleep, and fall asleep in the oddest and most uncomfortable positions because I cannot fight it. I can be watching a stimulating video or film with pretty decent volume, or listening to loud -ish music, but I'm gone, dreaming, and don't hear a thing. I usually enter that sleep no matter what and come out of it within half an hour feeling quite alert really. I always have to re-boot the film though because I have missed so much of it!
I slept through almost all of Beethoven's ninth symphony after dinner once, and that is pretty noisy!
I am not sure what's the cause. In my own case it doesn't seem to be blood glucose problems, as I don't get any other blood sugar issues, plus my blood tests never show anything pre-diabetes etc. but I think it may be because I have had an active physical day, and often a slight sleep deprivation the night before. Then I eat dinner. And the body just wants to "rest and digest". So it flops entirely to get that done. But I am an older person. I accept it because of that.
It doesn't seem to be any food intolerance either with me. It happens when I eat foods (very simple foods) that suit me well.
The more decent sleep I had the night before, the less likely that is to happen.
But for you, maybe it might be worth mentioning this to your doctor and getting a blood test run? Just to check that things are OK? Your blood glucose levels and liver enzymes etc.