Am I obligated to compensate for what I secretly ate from my grandmother's food in my childhood, considering the different sources of funding for household expenses, especially since I do not remember if that was before or after puberty?

There is no sin upon you for eating the food of your grandmother, because common practice dictates leniency in such matters. Allah Almighty has permitted this, saying: "Nor [is there blame] upon yourselves if you eat from your [own] houses or the houses of your fathers or the houses of your mothers" (An-Nur:61). As for obsessive thoughts (waswas), their cure lies in turning away from them and not paying them any attention.