well here's the bad news..
There is always the possibility that she could be pregnant, during any act of penetration. This is especially if your penis did rub against her vagina and you had previously ejaculated. And, there are possibilities that she may get pregnant even if you didn't "slip-out," because your semen could have leaked out of her anus and down her perineum and into her vagina, if you came inside of her.
however, there is good news..
I'm sure this happens with many couples, and the fact alone is nothing to get too worried about, unless the signs of pregnancy may occur. Talk to your girlfriend about her period, and if it comes unusually late, sorry, that's a bad sign. Not always accurate, but bad. I'm not very old but I've had sex with my boyfriend, and although he is older than me, I tend to find myself telling him what is safe before hand, because i tend to have little conscience "during the act." My parents are nurses, and trust me, I've been the worried one too. It's not a good feeling. But like i said, unless the signs are bad, such as she misses her period, she starts to get morning sickness, etc, then cool off :) Stay calm, and don't let it affect your relationship. Try a pregnancy test if your really worried if she misses her period. My BEST advice though, is to always do it with a condom, even if there isn't vaginal penetration. It's not very fun, I know, but better safe then sorry, 'til you know you don't have to be safe [when your ready to have kids..] ;)
hope this helped, sorry it's so long =/
Yes, there's a possibility your girlfriend is pregnant. Why? Because prior to orgasm, your body releases a fluid to clean out the urethra of any remaining uric acid. This fluid can contain millions of sperm.
You girlfriend should take a home pregnancy test ASAP. If it's positive, there are many options to consider.
I don't recommend anal penetration without a condom because a) mistakes can happen--like the one you made; and b) bacteria from your penis can contain various pathogens which may be unfriendly to your girlfriend--either anally or vaginally. Some of the bacteria in your penis may be just fine for you, but not for her. Play hot; play safe; use a condom. Dr. J