Receiving with a Forgotten Mortal Sin

A sin is an act of the will. You did not purposely receive Communion unworthily in the above situation, so, while objectively wrong, you would not be culpable of sin for unworthy reception. You would, however, be obligated to confess your mortal sin and objectively unworthy reception of Communion as soon as reasonably possible.

The Code of Canon Law:

A person who is conscious of grave sin is not to celebrate Mass or receive the body of the Lord without previous sacramental confession unless there is a grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition which includes the resolution of confessing as soon as possible (can. 916).