Withdrawal bleed or implantation and pregnant

So I’m on my placebo pill week. I started my withdrawal bleed yesterday I think. It’s been enough that I need a tampon but the tampon doesn’t fill up. Yesterday I ruined a good pair of underwear but since then, there hasn’t been a ton. Could this be implantation bleeding and I be pregnant or is this definitely my period/withdrawal since I’m on my placebo pill week. I’ve heard of some people still getting their period and then find out they are pregnant two weeks later