What does an early short period mean?

My girlfriend has recently had intercourse with me, but we used a condom and we might have not put it on right away is the problem... but what truely worries me (were not ready to have a child) is that she is pregnant. She told me that she has experienced a shorter version of a period basically, but with less blood. It actually was just over the course of one day. She says there was nothing odd about the color of the blood... it was just really short and over quickly. What could this mean?