What would cause white color in menstrual blood clots?!

Background: I'm finishing up my first pack of orthotricyclen, just started the green placebo pills. I'm not currently or have ever been sexually active--I'm just getting my cycle straight for my wedding in two weeks. I've been on orthotricyclen before several years ago when I as on Acutane and did not have this issue then.

My flow started yesterday, as expected. However, it's been mainly consisting of large blood clots about the size of my thumb. They are also up to 50% white with a color and consistency reminiscent of cervical mucus. Other than the clots, there's not much flow.

I don't feel sick or particular crampy--in fact, I feel better than I usually do on periods.

So should the clots and the white color be a concern? And what's causing them? I've had menstrual clots every now and then, but not like this.