What Causes Clots And Heavy Bleeding During Menstruation?

Iv been on my period for 5dayd already with heavy light bleeding on and off but today (my5day) as i was changing my tampon i felt like something big wierd pass down my vagina i looked right away and there was a huge ball dark/light red blood clot looking.ive had clots befor during my period size smaller then a quarter but this clot was really huge almost like a tennis ball.what can that be?