Breakthrough Bleeding

I started extended cycle birth control pills about a month ago and I was warned about breakthrough bleeding. I just starting experiencing it a couple days ago. First it was light spotting then it got very heavy. I just went to the bathroom and noticed a huge thick blob of something. I investigated a little and noticed it was a lot of thin idk what but it was all one piece. When ever I'm on my period I've seen my uterine lining but it was always just a small amount every time I changed a pad. Can anyone explain this to me?