Chris,

First, subconcussive impacts have a cumulative effect when one suffers hundreds of them over a few months. One infrequent impact is not a big deal.

Did you fall sleep before or after your meal ? If after, it could be because you meal caused an increase in blood sugar. This can cause drowsiness and sleep.

A short bout of anxiety from the "Did I hurt myself bumping into the wall?" thought can also cause sleepiness.