If the situation is as you describe in the question, then you have sinned because this was disobedience and bad treatment towards her. You are guilty of sin since you knew that your mother was bothered by (that food) and that she had told you not to cook it. So you were sinning by doing that, and you were disobedient, severing the ties of kinship and disobeying your mother. You have to pay the diyah (blood money) because your act is counted as semi-deliberate killing. You also have to offer expiation (kafaarah), which is to free a believing slave; if you cannot do that, then you have to fast for two consecutive months, i.e., for sixty days. And you must also repent to Allaah.
We ask Allaah to accept your repentance and ours, and to help us to do all that is good.