Welcome to HCM.Pregnancy
is possible if unprotected sexual intercourse is done around ovulation time in regular cycle.
Your LMP was 15th Oct and did sex on 6th Nov. You have 28 days cycle. You have missed period for more than 1 and half months.
You do a blood test
for beta-hcg and USG lower abdomen to rule out pregnancy and other underlying pathology if any.
If pregnancy test is negative,then have to do some hormone study like LH/FSH,TSH, blood estrogen & progesterone
etc. Possibilities are hormonal imbalance, wt loss or gain,stress, ovarian cyst
,alter eating habit etc.
so consult with your gynecologist and take advice according to test reports.