Bleeding 8 days after sex??

Abi

So we had sex on the 8th. He came inside me... the next day I took plan b and then on the 17th I started bleeding.. I’m not due for my period until the 26th or 28th... I went to the dr and they said it could either be a side effect from the pill or implantation bleeding. Who has experienced implantation bleeding and what was it like? When did you test positive for pregnancy test??