What is a sin?
What is a sin? There is an original sin: do not love yourself. Other sins, they are known to each one of us. You always know when we are committing a sin and when not.
On the day of Judgment we will be shown moment by moment our lives and it will be ourselves to recognize what has been correct and what not.
In the presence of the Truth, of God, the lie is pulverized, only the truth survives.
If you feel guilty, just ask forgiveness. If you are not for something real, but you feel guilty anyway, as it happens to many, it is because you keeps alive the original sin, which is not to love yourself.