Implantation or start of period?

When I pulled put my tampon there was a small clot. (Didn’t feel comfortable posting the pic)

Is this associated with implantation or period? The blood was a mix of old dark brown and bright red

Background story: I started my placebo pills on Sunday. Starting spotting that night. It was dark brown and filled a tampon. It got a bit lighter the next day but red blood was present. Took a pregnancy test (negative). I’m still bleeding enough to use a tampon and the red blood as been more noticeable. I’m supposed to officially start today.