Etiquette question: Eating guests' food

Dear family, I have a question for you. I have guests coming over for a potluck this evening for church. I told them I would make macaroni and cheese and I did. However, it was very late last night when I made it and I was very hungry...so I ate some of it. I felt guilty even even as I dug my fork in. I'm not sure, but it seems like when you come a potluck, you expect the food to be undisturbed. So, what do I do? It's, of course, baked mac and cheese, so the fact that the corner is missing is quite obvious. Am I just being a freak or is this in very poor taste?

E