I guess it may be a combination of things. I for one used to be always drowsy after lunch, then i went vegan 5 yrs ago and stopped feeling that way. Then 3 months ago i started to exercise. Prior to this my exercise consisted of walking to my car, sex, and nothing else. At some point shorlty after i increase physical activity i started to feel drowsy post-lunch again (very!), but as i've continued to exercise i've also taken a hard look at my diet and now i've left out most simple carbs (brown rice instead of white, whole grain bread instead of white, etc.), i've now included a lot more vegetables/fruits/grains, and i also try to eat smaller portions but more frequently. I'm also paying more attention to getting more protein (oats, soy, quinoa, etc.), and especially i pay more attention at what i eat pre/post workout/matches. I also mind sugar intake a lot more (the skittles had to go
) Post workout is the only time when i sometimes will eat refined flours. Initially i also made the mistake of eliminating all fats, but have now reincoporated them in the form of avocados, peanut butter, olives, pistachios/other nuts, tahini, flaxseed, coconut milk, etc.
I'll still sometimes feel sleepy if i'm sitting at home watching TV in the afternoon, but i think that's also related to me getting only ~5 hrs of sleep at night (need to fix that i guess). Finally, as i type this and look around my desk, everyone looks sleepy or has completely nodded off, i may be the only one fully awake, so i guess it's a 'normal' thing, don't feel too bad about it.
On the other hand, if your lunch consists of 4 toasted whole grain bread slices and nothing else, from one vegan to another i urge you to get a far more balanced diet, otherwise soon you'll be one of the 'i had to stop my vegan diet due to health issues' and probably start a blog about it that will get comments to the tune of 'i'm so happy for you, you're so brave and it's great you listened to your body' silly comments.