What Causes Grayish Vaginal Discharge With Stomach Cramps?

I am on seasonique and I've had some breakthrough bleeding this week. Yesterday I had some really painful cramps and when I went to the bathroom there was a grey tissue discharge on the toilet paper. It was sticky and had some blood on it. It was about the size of a half dollar. After this passed the cramping stopped. I'm not sure what happened or what I should do. Any help would be appreciated.