Particularly large meals take time to digest, meaning blood may be diverted away from other body areas to help with this. Further, if you are dehydrated during or after eating, this may exacerbate your lethargy.
The food you have eaten is diverting your blood for the digestion process. While natural, depending on what you eat, this process can increase your energy levels, or it can cause sluggishness.
The contents of your lunch will have a large impact on your afternoon's energy levels.
Keep hydrated and keep a food diary for a couple of weeks, note which foods make you the most drowsy. Then take the diary to a GP appointment for further tests and discussion about the foods you are eating and the impact that they are having.
As you say, your other conditions are being regularly monitored so may not be linked.
Keep us posted.
Perhaps you are from Spanish decent and you need a siesta!!!
Let us know how you get on.