period came back??

hi ladies ! so my period was from last monday to last friday, a total of 5 days. while i was at work this morning i felt something come out. thinking my period was finished, i expected discharge seeing at i'm going to be at my peak of ovulation. well, i went to the bathroom and to my surprise, there was blood. last night and the night before i had sex... rough sex really. i'm not sure if this is normal. pregnancy is out of the question seeing as i took a test to make sure and it was negative. i just wanna know how long this will last and what's causing it ??? i'm usually regular.