Answer:
yes!
However unlikely it is still technically possible
If you think this may have happened you might not want to play around. If you have access to something like a day after pill, you may want to use it (if you are not otherwise morally opposed)
Again I am no expert but I remember reading that it takes a certain amount of semen to have a good chance conception, but really it depends on how much got in.
I think this situation is unlikely BUT there is a good chance it could happen so be careful!!
Anal sex is like someone being offered a piece of meat that is good and fresh and another which is rotten, then he chooses to eat the rotten piece and leaves the good one.