How come women can only get pregnant after their period?? Why do women get their period??

What do you mean "women can only get pregnant after their period"? Young girls can actually get pregnant even before they're first period because sometimes ovulation occurs without a period. And you're most likely to get pregnant "between" your period when you're ovulating but a woman can actually get pregnant at anytime.

Yes, it's unusual but pregnancy can occur during your period, or before you have ever had one. Women have periods, it's a drag, why don't animals have to wear pads, huh?

Every (approximately) 28 days, the human female of childbearing age goes through a cycle. We usually count the 28 days starting with the first day of our last menstrual period. 14 days after it starts, we ovulate (let an egg out). Sperm can live up to 4-5 days in the female body and can fertilize the egg 5 days before, to five days after ovulation. 14 days after ovulation a new menstrual period starts (if pregnancy did not occur).

We start bleeding, or menstruating, we shed the lining of the uterus. The lining builds up to make a nice home for the fertilized egg. If the egg does not get fertilized, the lining is shed again, along with the un-used egg about 28 days after we started bleeding before.