Why did Gamelin not tell his mother about the bread?

Times are tough in The Gods are Athirst. France is in the grip of war and revolutionary terror. As well as systemic repression, there is economic chaos, with skyrocketing prices and chronic food shortages. The Jacobin regime has implemented a number of draconian measures to help combat the various problems, for which they themselves are largely responsible. For one, they impose a nationwide bread ration so that everyone gets a share of the rapidly-dwindling food supply.
Évariste Gamelin, the protagonist of the story, is an idealistic young artist transformed into a fanatical Jacobin, ardently devoted to the Great Terror, in which he becomes such an enthusiastic participant. As a true believer in the Jacobin cause, Gamelin gladly waits his turn in the queue at the bakery until he finally receives his bread ration. It's the last loaf left in the shop; a small one weighing just less than two pounds.
On his way home, Gamelin sees an emaciated young lady nursing a small baby. The woman's in such a wretched state that it's tragically clear that she hasn't eaten in a while. Gamelin is moved by the woman's condition, so he takes out a knife and cuts his small loaf in two, leaving half of it behind for the starving young mother.
When he returns home, Gamelin lies to his mother, telling her that he was so hungry that he couldn't wait and so ate his half of the loaf. He gives her the other half. Gamelin doesn't tell his mother what really happened, presumably because he doesn't want her to get upset thinking that her son was going out without food. Perhaps she might also have been angry at her son giving his bread ration to someone he doesn't know all that well.