I guess you are worried about pregnancy right? (Just checking you know that you can get STIs like HIV from penis in anus and penis in vagina sex from someone with an infection)

It is possible for sperm to leak out of the anus, along the perineum (the bit between the anus and vagina) and then go into the vagina. The likelihood of this increases I guess if you were facing away from your partner. If you had anal sex in a facing position with you on your back, then gravity will take the sperm in the opposite direction to your vagina. Your bf could have transferred some sperm into your vagina through fingering too.

It is possible to get pregnant this way, but it isn't likely. Sperm needs all the help it can get to fertilise an egg as it's a pretty difficult job. It needs to be ejaculated directly into the vagina at the right time in a woman's cycle. Even then it's not a gimmie that one of the millions of sperm (50 – 500 million) will fertilise an egg. More on this here http://bishuk.com/2010/09/14/female-fertility-explained

However I'm not saying that it's safe. Using condoms will prevent the risk of this happening and are also great at preventing infections.

Hope this helps

Bish