Not impossible. Although it is unlikely that you would get pregnant on your period, it is not entirely impossible.
Not exactly. Women can get pregnant when they are bleeding especially when the bleeding is not their normal period. Not all vaginal bleeding means they are on their menstrual cycle. Ovulation occurs 14 days before the cycle starts and this is the optimal time to get pregnant. When a true menstrual cycle comes it means ovulation did not occur. Having sex during that time would be low risk for pregnancy.
Unlikely, but... In order to get pregnant, intercourse must occur around the time of ovulation (releasing an egg). Ovulation usually takes place about two weeks after menstruation, which makes it unlikely (but not 100% impossible) you would become pregnant from intercourse around the time of your period. If you are trying to avoid pregnancy, be sure to discuss effective birth control options with your doctor.