Thanks for the query.
According to your menstrual history there is very less chance for pregnancy
as ovum usually won't release during around 7th day in case of regular periods.
If you are having irregular periods
you can take the white pills in the schedule as you mentioned.
You may have side effects like nausea
etc. for it. By taking antacids and anti-emetics the symptoms can be decreased to some extent.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429