You cannot get pregnant from the act of anal sex, however, because of how close the anus is to the vagina, semen can drip down to the vagina, enter it, and travel - resulting in pregnancy.
To reiterate what Ashelen said, NEVER put your penis in her mouth or her vagina after anal intercourse, unless you are dead set on getting her pretty ill. The amount of bacteria in the anal canal is crazy, and it takes very little to cause infection.
Unfortunately, you are posting on a forum where many women are against abortions and Plan B as well. If your question is can you use Plan B, then the answer is sure, you CAN do anything you want, regardless of what we tell you. If your question is should you use Plan B, then most of us on here will answer a concise, "no." Like Joy stated, you were responsible enough to have sex, so you should be responsible enough to handle the consequences. If you don't want a baby right now, find another method of birth control - condoms, foam, pill, etc. Don't rely on Plan B. Plan B will not stop pregnancy if implantation has already occurred, however, it can cause birth defects if implantation has already occurred.
You can get pregnant via anal sex. Semen can seep down and a tampon isn't going to keep sperm out either (they are microscopic).
I do not recommend Plan B. If you are going to be responsible enough to have sex you should be responsible enough to handle the consequences.
Hummm that's a pretty complicated situation....I would say the chance of pregnancy is fairly low just because of the combination of circumstances, but there's a chance with anal sex just because of the closeness of the anatomy. I would say wait to see if her period is late and test, and if you really want you can take an emergency contraceptive.... Is she on any sort of birth control? it's a good idea, because in the future anal sex can definitely get her pregnant from what I understand, even if you try to practice the pull-out method. Also I would caution you both..do NOT let her put her mouth on your penis after you have been having anal sex. there are all sorts of germs and bacteria that you do NOT want to transfer between orifices (anal, vaginal, oral) without a complete and thorough cleaning, not likely to occur in the heat of the moment. and whatever you do, do NOT *repeat, DO NOT* put your penis in her vagina after it's been in her anus. this is BAD bad bad. But I would say...if you're going to be constantly worried until her period arrives, go ahead and get a Plan B, but in this case you guys probably are alright.