You are only obliged to confess mortal sins. The sin you engaged in was grave matter, meaning it was serious. But it wouldn’t be mortal for you if you truly didn’t know it was grave matter. However, confession also gives us grace to not commit particular sins. So, it is to your benefit to confess a sin that was grave matter.
If I were you, as uncomfortable as it would be, I’d confess this serious sin next time. You don’t need to be graphic about the sin. In fact you shouldn’t be. I would maybe say ‘I committed an unnatural sex act’ if that expresses correctly the sin. If the priest needs more info he’ll ask.