I had mild spotting early in my pregnancy. My OB said there were several reasons:
most likely in my case was that the blood vessels around the cervix are so full, that any awkward positons (like getting up from a sitting position awkwardly or being constipated and 'bearing down too much' (sorry if TMI) can actually rupture some of them. Also, she said that some women can have what's called 'breakthrough bleeding' when their period would have normally ocurred early in the pregnancy.

As long as the blood is somewhat brownish (and therefore old), and in small amounts, it is nothing to be too worried about UNLESS it happens frequently. If you have any bright blood, clots or large amounts associated with cramping (especially if it's one sided), you should consult a doctor or your OB.

Once I'd had a chat with my OB about it, I felt far more reassured, and also talking to other women (like on this forum) helped a lot.

The main thing to remember is that everyone is different however, and you should follow your instincts. My OB always recommended having the next day off if I got a small bleed, to get off my feet etc.

Good luck!

Julesy