With all due respect ... if one places oneself under an authority, government, Church, employment, what have you ... then to go against it willfully is rebellion, if nothing else. In some cases rebellion is necessary, such as if a person were ordered to do something that goes against God's law.
But, and I REALLY don't want to get into arguing here, although I know that every group has their own ideas about what Communion is, and how it should be handled, and who can receive it, there are reasonable prescendents and history that go back to the earliest Church that form the foundation and reasons for what the Traditional churches do in this respect. Suffice it to say that going to a particular church and DELIBERATELY flouting their ways of doing things, would in fact be a sin. I don't think the OP did so deliberately.
For that matter, the definition of "sin" has become changed. The western view tends to consider "sin" as only "breaking God's law" But in fact, the widest and oldest definition of sin/hamartia is that of an archer who has aimed at something, and missed his mark. He might miss it by only a very little, or a great deal, but he still misses. Hamartia is that which falls short of the goal, and the goal ought to be Christ Himself. So in fact, we all sin constantly. God in His graciousness only makes us aware of how far we fall short to the degree that we can handle the knowledge, responding by confessing and trying to become more like Christ, rather than falling into despair if we saw ALL of our shortcomings.
But I'm not looking to argue.
Being that this was placed in denomination-specific theology, though, it's not helpful to criticize one's chosen denomination and overlay another understanding upon their question.
And by the way, I respect you very much sir. I only see a portion of your posts on CF, but you have written things I have found helpful and inspiring. This reply is by no means a criticism of you personally.
God be with you!