I think the best way to find out would be to test your blood glucose when you start to feel sleepy.After eating a meal not very carby I often fell very sleepy is that to do with Diabetes.
ThanksI second @Pipp 's advice- testing your levels will let you know whether it is related.
However, if it turns out not to be related I suggest mentioning it to your doctor at some stage. I would suggest making a note of it- what you ate, what time of day it is, how you slept etc. All things which may be relevant. You may even notice a pattern yourself.