is this a possibility?

KK ladies, so, my boyfriend and I had sex a few times during my ovulation but one particular time i truly believe he came in me. NORMALLY he doesn’t, we use the pull-out method and has been working for almost a year. my ‘period’ came 4 days earlier than it should have; i put “period” in quotes because i’m not sure if it IS, given how light it’s flowing at this moment. usually, i bleed heavy. could this be implantation bleeding? does implantation bleeding happen before or after your missed period?