A lighter, shorter bleed than your usual period, especially after unprotected sex earlier that cycle, can sometimes be implantation spotting rather than a true period. It can also just be a one-off lighter cycle from stress or a hormonal dip, which happens to most women occasionally.
Next Steps
Take a home urine
pregnancy test now with first morning urine, and repeat it in a few days if your period for this month doesn't show up as expected.
Health Tips
Please see a gynaecologist if you get pelvic pain, unusually heavy bleeding, or if your cycles stay irregular for another month.