is possible only,if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period. Ovulation occurs approx 14 days before due date. Other days have low risk or safe period but it varies & has high failure rate.
is mini pill with progesterone
only with effectiveness more than 99%. Do a urine/blood test
for hcg to confirm/exclude pregnancy if you missed period more than 1 wk.
If you get negative test, then need to do USG lower abdomen and some hormone tests to rule out underlying pathology if any.
imbalance,wt gain/loss,thyroid prob,irregular eating habit may causes delayed period. So consult with your doctor to take suggestive measures.
You may not need OCP if your husband is leaving for long duration.