It's partly allergic reaction, partly sugar/insulin reaction. :) Examine your lunch - did it have high glycemic index foods with simple sugars? starch? Or was it heavy/processed/fatty?
I hope you'e not getting tired after EVery meal! :) When my liver wants to let me know it's time for maintenence I get the post-meal sleepies, but I only ever get them after fatty/heavy or sugary meals. I tend to be wide awake after my veggie juice, which is why I tend to have that for lunch. It's hard enough staying awake at the office without adding a heavy lunch into the mix LOL!