You are just 15 yrs old and did unprotected sex with pulled out method. It has high failure rate but depends on ovulation time.
As your period due date was on 18th Dec,so ovulation occurred on 4th Dec and unsafe period was 2nd to 7th Dec.
Now get period with mild spotting.It is better to do urine/blood test
for hcg to confirm/exclude pregnancy
If negative,then you may wait for 1 wk and it may be due to hormonal
,emotional upset etc. Here need not to be much worried.
If you still not get period,then consult with your doctor and need some investigations like USG pelvis etc and take suggestive advice.
Avoid stress by deep breathing/yoga
,control body wt,take healthy diet and proper sleep useful.