On my second year on nexplanon this december 2018 and unexpectedly missed a period i did have spotting in the beginning of month can i be pregnant?

Yes but unlikely : as Nexplanon is over 99% effective when used correctly (used for 3 years max). To reassure yourself, take a urine pregnancy test. Hormonal birth control can cause irregular bleeding such as missed periods and spotting. Also make sure you are using a barrier method as well (like condoms) as this is the only way to prevent STDs!