I don't want to be tactless, but could it be a miscarriage? This happened to me once, and it was a bit later than usual too. It went on for around two weeks. A few months later, I found out I was pregnant, then miscarried at 12 weeks. It was only because I knew I HAD been pregnant that I knew it was a miscarriage, if you see what I mean. I realised then, that what had happened before was the same - I had miscarried without knowing. Just a thought.
I'd definitely see your doctor if it's unusual for you xx