I told my Dr. that I had called during visits because I was kind of spotting light pink. He said that nothing scares a pregnant woman more than seeing a little blood. He assured me that there are so many reasons you could spot for a bit and basically said don't worry. BUt, ditto what Amanda says if there is a change.
I have been having that off and on since 4 weeks. I had an ultrasound monday at 6 weeks, and everything was perfect, including the heartbeat. It happens and usually it's nothing to worry about...it's actually a whole lot more common then you probably think. As long as it stays light, doesn't turn bright red or come with clots or cramps, you are probably fine. If it gets heavier, give your Dr. a call. Good luck!