Why do I feel my schoolbag to be heavier after meals?
Could certainly be physiological. After eating more blood is diverted towards the smooth muscles around organs for digestion. This will mean your skeletel muscles have decreased blood flow so less nutrients such as Glucose available to make contractions. As you have less energy available to contract the skeletal mucles you're more than likely to feel slightly weaker.

On the contrary it may he purely psychological! :) Good question though!