It seems to me that you did not deliberately withhold a serious sin, but were interrupted and not afforded the opportunity to confess. If what you had left to confess was venial, then don’t worry about it at all. If what you had left to confess was mortal, you still didn’t deliberately choose not to confess it, you were just not given the opportunity to do so. Mention it at your next confession, but don’t let it stop you from receiving communion.
I would argue that it has the same moral weight as not remembering to confess a grave sin. Your memory prevented you, you didn’t deliberately choose not to confess it. In this case, the circumstances, namely the interruption and the subsequent flusteredness that likely resulted, prevented you.
-Fr ACEGC