| Bizarre, I had this the cycle before I conceived and had no idea what it was.
I had it for about 3/4 days, and it was only a few days after my last period finished. It was too early to be implantation bleeding. But I had just come off contraception and so when I spoke to a nurse, she said it was just my body getting used to getting back to normal.
Not sure if you've just started TTC but if you've just come off something, that might be the reason. I has one period after this happened and then I conceived.
Although, of course, it could always be implantation - you don't always ovulate exactly when you think you do. | 
