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Anyone know what the procedure is if you recall a sin you're unsure of before receiving communion? It was after confession and I forgot it so I thought it would be fine. Now I'm having second thoughts.

If you think that you might possibly have committed a mortal sin and that this does not arise from merely a scrupulous disorder, then you must not receive Holy Communion under pain of committing a sacrilege. Don't receive Holy Communion.

I think I explained badly. I was asking if, after having gone to confession preceding mass, and while sitting at the pews, you recall a sin, but one that you're unsure of and intended to ask the confessor about. And If at that time, reasoning the forgotten sin would have been forgiven, if it is acceptable to receive communion or not.

First, the traditional teaching is that a valid confession forgives all mortal sins, including those genuinely forgotten. The penitent has the obligation to mention the forgotten sin at their next confession once they remember it, but the sin itself is already forgiven.

Second, it is always acceptable to receive Holy Communion, unless you are truly conscious of unconfessed mortal sin. You cannot be guilty of mortal sin (and, therefore, in a state of mortal sin) without knowing whether the matter is grave/serious/objectively mortal to begin with. This is what you wanted to ask the priest about. Therefore, because you were unsure that the matter was mortal, then by definition, you could not be guilty of mortal sin in that particular instance. So it was perfectly acceptable for you to receive Holy Communion.

Mortal sin is thumbing your nose at God. A person committing a mortal sin knows that God has forbidden that act but he decides to do it anyway. A person who is honestly ignorant about the seriousness of the sin (in God's eyes) does not commit a mortal sin. The sin would be venial, in that case. Venial sin is still very bad and should always be avoided, but it should not keep you from receiving Holy Communion.