The way you describe it you sound like you may not have really reflected on the sinful nature of the action and made a personal choice to do it anyway. If so it would have been a venial sin despite being grave matter.
That said, it can be very dangerous business trying to decide for yourself whether you met the subjective requirements for mortal sin when you have committed an objectively gravely sinful action. It’s easy to talk yourself into believing it was not a mortal sin, then later on suffering from a nagging doubt about whether you are in a state of grace, receiving the Eucharist worthily or not, etc. That’s not a good way to live, with the possibility of damnation constantly hanging over your head. Then there is the possibility of committing the sin again, more or less willfully keeping yourself in “autopilot,” because you essentially got away with it the first time. It’s highly unlikely, in my opinion, that you could do that for long without truly committing a mortal sin.
For this reason I would recommend confessing the sin before receiving communion again, whether it was actually mortal sin or only venial.
Obviously don’t let this advice overrule any direction your priest has given you though. For example if you suffer from what is commonly called “scrupulosity” then a less, well, scrupulous policy towards confessing sins whose gravity you are unsure of might be wiser. If you are like me though your natural inclination is in the opposite direction, and we have to be tougher with ourselves than we are initially inclined to be.
