Bleeding cd 12?

Ashley
I'm on cd 12 today. Usually I have long cycles, 38-40 days. When I bled twelve days ago, it was light and only lasted two days instead of three and a half. It came early. Thought maybe implantation bleeding, but a hpt was negative. Now today, out of nowhere... Blood. I hadn't even started opks yet for the month since my cycles are so long. Any thoughts?