If there's any truth to your anonymously posted story, go on birth control pills if you are so worried about pregnancy (a good worry to have) and want to have unprotected sex. Also, get to the library and read up in all areas of sex education because you clearly have a serious lacking in your understanding about it. As others have said, if there is a fistula (a tunnel) between your rectum and vagina, sperm could get in there that way and possibly cause pregnancy, but you would have much more serious concerns about the fistula considering the major risk of bacterial infections it would cause, and you'd probably be seeing a doctor about it already. And if semen leaked out of your anus and ran down to your vagina (you'd have to be laying on your stomach for that), there's a slim chance it could work its way inside. But even if semen did get inside your vagina, you'd have to be currently ovulating and fertile in order to get pregnant. If there's no egg present then you are not fertile and a gallon of semen in the vagina couldn't get you pregnant.
So overall NO, anal sex won't get you pregnant. Provided you have a healthy rectum, the semen will be nicely contained inside and can't get through the walls of the rectum to get to the vagina. That's what's so nice about anal sex: it's immensely pleasurable and intimate like vaginal sex and can enjoy all the eroticism of internal ejaculation without the risk of pregnancy.