Could just be an off cycle. Could be "implantation bleeding". This isnt typical but can happen - when a fertilized egg plants itself, some bleeding can occur. It could be a "missed abortion" or chemical pregnancy which is when an improperly formed egg or sperm is fertilized but cannot grow beyond a few days.
You could try another pregnancy test or schedule a visit to your doc to find out whats going on...but doesnt sound like something to worry over.