6dpo, lower back pain and cramps. What is going on?

I'm really confused and don't know what to think. I temp and use opk's so I know I'm only 6dpo. I'm having moderate cramping (mainly on the right) as if AF is starting. The lowest part of my back is aching. AF isn't due until 3/8 and I've never had cramps this early. What could be going on?