The scene I'm picturing from your description would leave no greater risk for pregnancy than any other time using a condom.
The way I'm reading this, you pulled out, the condom came partway out with your penis, but then slipped off leaving the tip of the condom, but not the opening, inside her for a few seconds. If that's the case, then there was never time for anything inside the condom (like any spare sperm floating around in the precum) to come in contact with her vagina.
If the mouth of the condom was inside her when it slipped off, then there's a small risk involved, but it's really small. While precum can get you pregnant, people using the pull out method PERFECTLY have a pregnancy rate of 4% per year. In other words, if you're only exposed to precum, your average risk of pregnancy is 4% in a year. If the condom was spermicidal, then I think the risk is basically non-existant.
As for the period, hopefully it's just a little anomaly in her cycle. If it's more than a week late, then a pregnancy test should be accurate and you'll have given it time to come if it was just being a little finicky. Depending on how old your girlfriend is, and how regular her cycle is, her period could be fairly irregular, so if you end up with no period but a negative test, that could be the cause. And you can always go to the doctor for a blood test or if you have more questions.
Some condoms come lubricated with spermicide. If the condom you used was one of those, it would have killed off any sperm.
If your condom was one that did not have a spermicide lubricant, there is a risk of pregnancy.
There are more reasons that pregnancy for a woman to be late with a period.
A pregnancy test will be able to give you the answer.
You can get different sized condoms, but it is great that you are responsible for trying to avoid an unwanted pregnancy.
Best wishes
no I don't think she is pregnant. but if her cycle is two days late. take a pregnancy test.