The first thing you need to do is educate yourself on how the woman's reproductive system works. A woman can conceive only within a short phase of her monthly cycle (between two periods). It is the time when ovulation takes place, and the ovary releases an egg, which can get fertilised and develop into a baby. The rest of the time, she is not fertile and cannot conceive.
The days immediately before and after menstruation are considered as a safe period. You did not require the Unwanted 72, on the last day of your period, because intercourse would not have resulted in a pregnancy.
Pregnancy can happen only when a woman ovulates. And women ovulate just once, between two monthly periods. If she conceives, she misses her monthly period, until the baby is born. Pregnancy can occur within a day or two of the egg (ovum) being released. It is because sperms (from the male) can stay alive for up to 72 hours. If a woman has unprotected sex a day or two before ovulation, she still has a chance of conceiving, because live sperms will still be present in her vagina.
It is easy to determine the days to avoid sex or take precautions. Ovulation usually occurs on the 12th-14th day of your monthly cycle. You begin counting the days from the first day of your period. If your periods start today (04-10-2019) and you have a regular 28-day cycle, you will ovulate between 15-10-2019 and 17-10-2019. Keep in mind the sperm stays alive for around 72 hours, so protection is required three days before your ovulation date. The ovum or egg is fertile for 24 hours.
This calculation works if you have a regular menstrual cycle of around 28 days. The days can fluctuate between 26-30days, and it is considered a normal cycle. It is difficult to predict the ovulation period in women who have irregular cycles.
An easy way to know that you are ovulating is to check your vaginal secretion. It tends to increase and is clear and sticky, much like the white of an egg.
Coming to the spotting, you are experiencing. Contraceptive drugs affect your hormones, which is why you do not conceive when taking these pills. So, yes, it will alter the pattern of your periods, and the spotting you are facing is normal because the hormones in the drug are working.
'A love affair with knowledge will never end in heartbreak' - Micheal Garrett Marino