Thankyou for your response it has helped me a great deal!
I doubt very much if it is syphillis, it does sound more like a reaction to when you were shaving. Keep an eye on the spot and if it becomes inflammed or pus filled then a you may need some antibiotics. If you are going to practice anal sex then it is best to do it safely and use a condom, there is a lot of bacteria in the anus which could cause a nasty infection (I'm not talking about STD's here) which would be very uncomfortable. Also make sure that you wash thoroughly afterwards and do not insert your penis into her vagina following anal sex as this will transfer bacteria also. Hope this helped.