That's how I found out I was pregnant a few weeks ago. I started spotting the day of my period which is normal before the heavy bleeding but it never came. I spotted for three weeks. After two weeks I went to the er and found out I'm pregnant. It was because of a progesterone deficiency my doc said.
It could mean anything. Since you are posting in the pregnancy forum I assume you think that you might be pregnant so you will need to test at the appropriate time.