You are constantly in a menstrual cycle. It is the 28 days from the start of one period to the start of another. What I think you meant to ask is "Can you get pregnant during Menstruation?"...
The Ova is released from the Ovary and let into the Fallopian tube...if the Ova isn't fertilized it carries on through it's journey into the uterus where it gets stuck onto the side of the uterus and...
On a 28 day cycle its usually day 14 which is your ovulation day. You will know when your ovulating because you will experience egg white vaginal discharge.