Ay, mahirap talaga ang uncertainty na yan, sis! Kasi nga if it's lighter than your usual period and shorter din (just Sat afternoon to Monday morning), that could be implantation bleeding if you're sexually active, or it could just be an off cycle, marami sa amin ang nag-experience ng irregular spotting out of nowhere. Best move is to take a pregnancy test after a few days para sure, or mag-check ka na sa doc kung gusto mo ng peace of mind, lalo na if this is really unusual for your body. One of my mum friends waited three days before testing and it gave her a clearer result, so timing helps din!