My opinion is like Lesley27 says, it sounds like ovulation, pregnancy is most unlikely to happen.
Now another thing you need to watch for is for HPV which is the Human Papilloma Virus, that causes cervical cancer. Just because you have anal sex it dont mean you can't catch infections. Remember most of the anal acts happen on the "doggy style" pose, so your vagina is pretty exposed to any leak from your anal canal... so next time you have anal sex just protect your vagina opening. Just a friendly reminder. Take care and I hope whatever is going on is just your period coming soon... God bless.
my guess would be signs of ovulation.. are you supposed to get your period in about 2 weekish? (well in about a week now)
the fact that you havent actually had sex makes it pretty slim chanced that you are pregnant, you can take a prego test at the time that your period would be late to ease you mind if you want...