As per your history, you seem to be having irregular cycles recently. If you were having regular cycles previously, the cause of this irregularity should be found out first. For this, you need to see a gynecologist and get examined and investigated. You may need hormonal
profile and pelvic sonogram. If you are not ovulating naturally now, you may need induction. The period for conception will be around the day of ovulation as the ovum remains viable for one day after ovulation. So, in a 28-35 day cycle, the period for conception will range from 11-20 days. Please get further advice from your doctor. Take care.