Late or pregnant?

According to on glow I am supposed to be on my period. Me and well now an ex had sex a week ago on Tuesday. Is there a possibility that it's late because of that, or could be stress? I stopped taking the pill about a month ago. And we didn't use any protection when we did have sex. Should I wait to take a pregnancy test or should I take one now? I know it could be a million of things but just concerned