You did unprotected sex after period i.e in safe period with taking unwanted 72 on 15th Dec
in correct time.The bleeding occurs due to withdrawal effect of the emergency pill (unwanted 72) and it is quite common.
chance is almost nil.So don't be much worried and stay cool. This bleeding stops automatically and if persists for more than 5 days,then consult with gynecologist to rule out underlying pathology if any.
Pregnancy test is done with urine for hcg
and two pink line indicates positive test whether one pink line indicates negative result.
Wait for the next coming period and if missed do pregnancy test after 1 wk.
Practice safe sex
in future. Be well.