Weird period on tri previfem

So, I've been on birth control for six months now and have always had a moderate to heavy flow with minimal cramping.. This month I've had horrible cramping and a light period, I can't even use a light tampon. I have also taken this birth control before for a year and a half and this never happened. (I'm on tri previfem) I took a pregnancy test a couple days before period and it was negative.. Any idea why this is happening?