What Causes Spotting In The Mid Of Menstruation Cycle?

I had my regular period about 2 and half weeks ago for about 7 or 8 days,now today I started to spot at first it was a light brown then It gradually got dark,now Its red,with lots of clots,I m only 18 and i never had sex,this has happened a couple times but it s never been like this,what could be causing this and is their maybe something wrong with me?