no, if you did not think it was mortal then it is not a sin to receive, and more than that, if you did not know a sin was mortal at the time you committed it, but then found out later it was mortal, for you it was still only venial, and you can still receive communion,
it’s just important to avoid all sin period, mortal or venial, and to educate yourself on what is mortally and venially sinful, because to neglect learning your sins is also a sin in itself.
just keep in mind, that God is not so much offended with us when we fall from weakness as He is when we fall on purpose, and that goes for both mortal and venial sins, so once you discover something to be sinful, you should strive(by the grace of God) never to willingly commit any sin.
although in confession we should confess our venial sins to, and even if we do not have any mortal sins, it is profitable to your soul to confess both venial sins and also temptations to sin, because by confessing your venial sins while you are in the state of grace, you receive extraordinary graces which you can only get in confession that will help you to better overcome your sins, and to confess your temptations is also very profitable for you because by doing that you really knock the wind out of the temptaions sails.
alright? hope this helps, take care