I had exactly the same thing a couple of days ago. Really light brown spotting--only lasted about an hour and wasn't very much.
I had two MC before DS and both started with some brown spotting. But it was much heavier and didn't just go away.
I also had a bit of spotting with DS, but it was also at 10 weeks and was a few days after DH and I DTD. I was told that sex or exercise can cause some light bleeding around the cervix that shows up as brown spotting.
So, I guess it can go both ways. But if it's light and follows sex or exercise, and goes away, I wouldn't worry about it.