I was supposed to start my period 5 days ago. I had unprotected sex 4 weeks ago. Today I spotted but its not heavy or a lot. What does that mean?

Hard to say. You seem to have a cycle that is off but likely not significant. A normal cycle has ovulation about 2 wks before menses. Your sex was about 3 wks before expected menses. Preg possible but unlikely based on dates. Suggest you see your gyn for tests if menses soon. Best wishes.