Could I be pregnant?

So me and my boyfriend had sex and he came in me, the next day I got my period but it only lasted 3 days when it usually last one whole week. After I ended my period I took a pregnancy test but it came out negative but I feel like I took it to early cause it was only a week that I had unprotected sex. My upcoming period date is suppose to be on the 1st of April.  How would I know if I really did get my period or was it implantation bleeding?