I agree with kindergal, it could mean a lot of things. I've had breakthrough bleeding several times over the years-the first few times it was due to polyps which hd to be removed, the next few times it was because of low progesterone. And my ob-gyn said that sometimes your cycles change with age. You should probably have it checked out.
It could mean lots of things...You may have some breakthrough bleeding. Are you on any birth control or any other medication? Sometimes, that causes it.