I've just had nexplanon inserted and i had unprotected sex the same day. could i be pregnant?

Unfortunately maybe: If you had unprotected sex around the time you were ovulating ,that is, during your "fertile period" of the month, you could be pregnant. The Subdermal implant, Nexplanon, releases a progestin into your system which may take several weeks before it effectively prevents you from ovulating and prevents you from getting pregnant. Be safe; check your pregnancy test until you become protected.