How can I tell the difference between implantation bleeding and period bleeding?

I recently had unprotected sex on the 25th and 26th of July (no ejaculation). My last period started on the 2nd of July.

This cycle's period was 3 days late until this morning when I woke up and bled; a spot a little over the size of a dollar coin, no clot, just liquid.

I wore a tampon for an hour or so and, when I removed it, there was little new blood and plenty old, brown color blood.

Right now, I'm experiencing all the symptoms I usually do on the first day of my period.

What do I make of this?