Help?!

Hello, My name is Kae and I a 19 year old college sophomore. Me and my boyfriend just recently started having unprotected sex. I got kinda nervous the first time and took plan b.  Now about 4 days later I'm lightly bleeding and I've also had cramps like I'm on my period. My period ended the same day we had unprotected sex this is the same day I took the plan b.  I'm nervous if I'm pregnant cause my period just ended so why am I bleeding again?