In my eyes, this is a situation of “better safe than sorry”, so I don’t see anything wrong with it. If you’re not sure about the weight of a sin, better to assume it is worse than it is. That way you know for sure you are completely forgiven. There is no room for doubt. For example, sometimes an impure thought will pop into my head, and I fight it off but am not fully sure if I did enough to fight it. In that case, I count it as a full mortal sin during confession. Then I don’t have to worry whatsoever about the state of my soul.
On the other hand, deliberately minimizing the weight of a sin to make yourself look better would prevent forgiveness for the full weight of the sin. If it was just a mistake though, God’s not going to hold it against you. I agree with others too. Follow their advice to bring it up with your confessor.