The Eucharistic fast is not a matter of objective morality. A priest is freed from the required Eucharistic fast after he has celebrated his first Mass of the day (canon 919 §2) and those who are sick as well as those who care for them are also exempted from the fast (canon 919 §3). So it is not the mere presence of food in the stomach that would be immoral nor does it invalidate reception of the sacrament.
The point of the fast is to remind us that when we come forward to receive communion we are receiving something (indeed, someone!) extraordinary and therefore we should act accordingly. The fast causes us to form our routine that day around attending Mass, reminds us that “man does not live on bread alone” (Matthew 4:4), and is a minimal sign of penance for our sins which we need to repent of before receiving the Lord.
Whether someone committed a mortal sin would depend upon their attitude and motivations in breaking the fast. Someone who simply made a mistake or misunderstood the terms of the fast would not be in grave sin. Someone who flippantly disregarded appreciation for the gift of the Eucharist and the necessity to properly prepare oneself to receive Him would be gravely disrespectful and indeed risks mortal sin.
In your situation it was an honest mistake and therefore no mortal sin.
