Not to sound insensitive, but is this a joke? Wearing a condom during any sex (especially anal) is a good idea to prevent the transmission of STDS.
Pregnancy can only occur in the vagina, and barring some sort of freak hole (which would cause health problems your girl would know about, trust me) there is no passageway for your sperm to her vagina through the anus. Don't worry about it.
Generally when it comes to birth control you can only be aware about your own, unless you're administering her pills in person each time for the required term for their effectiveness, it's best not to assume that she's taking it regularly.
One possibility could be if she had manually retrieved your genetic sample from where you left it and put it in the proper place. However, the insidiousness required for that (along with the risks of bacterial infection associated with A>V contact) mean only a severely disturbed person would do it.
Assuming you have confidence in your GF's emotional-psychological stability and trust she would not do something so crazy, the best conclusion is that the vomiting and headache were caused by something else. Many forms of illness can cause these besides pregnancy, after all. She should see a doctor if it persists.
The best way to answer your question however is to take a pregnancy test. It doesn't seem very necessary consideing the circumstances but would be the best most scientific way to answer your question as to the source of these symptoms.
