What Causes Bright Red Watery Bleeding During Periods?

Ok so i had sex i believe 7 days ago, i used a condom but to be sure i took plan b the next day. i recently got my period yesterday but its not normal. its bright red and really watery. im also cramping really bad and im nauseous whats going on?!? im 18 i weigh about 150 and not much medical history