Hey Lvelesse,
You seem to be quite worried. I do not feel there is any reason for you to be under
stress of unwanted
pregnancy.
For pregnancy to occur, the sperms have to entre vagina. Then they will swim through the
uterus into the tubes to meet and fertilize ovum. After anal intercourse, there is hardly any chance for the sperms to enter the vagina and uterus. Please drop your stress.
Still if you are in doubt, you may perform urine pregnancy test and/or
ultrasonography 8-10 days after missed period; i.e. after 18th. Before that these tests may give false negative results.
If at all you turn out to be pregnant, your pregnancy can be terminated medically. Up to 9 weeks of pregnancy, use of Mifepristone and
Misoprostol jointly can very safely terminate the pregnancy. However, these medicines have to be taken by an Obstetrician's prescription and under supervision only.
I hope you have got the answer. I feel this message will remove your stress and you will get the menses in a couple of days.