Leftover Period or implantation bleeding?

On 8/26 (last day of my period), Me and my SO were doing foreplay and it went in without a condom. It went in only once and I freaked out and stopped. I got a plan B just in case too and I took it within like 3-5 hours I think. So anyways today (a week after the “intercourse” and the last day of my period), I used the restroom and I just got quite a bit of dark brown blood or discharge on my liner. I thought implantation bleeding was light pink blood or spotting. I don’t know if I should be worried about pregnancy.