It's probably some breakthrough bleeding. Keep taking the pill, and hopefully if your cramps are followed with some bleeding, it won't last too long. I am curious what you mean by late, though. A few hours? Several hours? What type of pill are you on?
I could just be your period. When I was on the pill and was late taking them, I got cramps before I started. I could be constipation or anything else really. If it keeps up or gets more painful, I would go see a doc