Nexplanon and scared im pregnant

Ok so I have been on nexplanon for over a month and a half now, I just had sex with a condom and he pulled out what are the chances I’m pregnant? I think I’m psyching myself out because now I’m experiencing back pain and stressed out. Also I don’t get my period on nexplanon when would be the right time to take a pregnancy test or how do I know I’m not pregnant without getting my period?