A lighter, shorter bleed starting with spotting the day after sex can be implantation bleeding, but it can also just be a one-off lighter cycle. Both are common and look similar.
Next Steps
A home urine
pregnancy test now will give you a clearer answer, ideally with your first urine in the morning. If it's negative, test again if your period doesn't arrive as expected this month.
Health Tips
See a gynaecologist promptly if you get pelvic pain, dizziness, or heavier bleeding than this.