I'm 25 And Have Been Getting Unexplained Blood Spots For

I'm 25 and have been getting unexplained blood spots for the past 3 days.I had my periods 12 days back. I have no pain and bleeding is not heavy. I went to a local clinic and my urine test was done and nothing abnormal was spotted. I am not pregnant. I'm worried about this unusual occurrence as this is the first time I am experiencing something like this. Could you please tell me what could be the reason for this.