Hellppp

My boyfriend and I had sex the day before I was supposed to start my period. He only ejaculated a little bit inside of me. (Weren't using a condom) So just to be safe we went to the store and got a plan B pill. I took it an hour after having sex but my period is late by 2 days?! What does this mean!! Please help and also my periods have been irregular but I'm just worried. Also I took a pregnancy test and it said negative 🤷🏼‍♀️