@ab Welcome to A2A. What's done is done, but pulling out is not an acceptable method of birth control. Many babies have been conceived that way. For pregnancy to occur, there must be a meeting of your sperm and her egg. From what you wrote, I would guess that your sperm was present and was deposited inside her. The only real question is whether or not she was ovulating, which is the release of the egg, and that usually occurs around the middle of her cycle. If she is on a 28 day cycle, for example, that would be about 14 days after her last period. Keep in mind that sperm cells, properly protected in her body can stay viable for several days.
Notice that I have said "about" several times. There is no way to pin this down to an exact day because there are variables that have to be considered.
You don't indicate how old each of you are but, unless you are prepared for a pregnancy, you might consider getting her to look at Plan B. This is an emergency contraception that must be take within 72 hours after unprotected sex, and the sooner the better.
Going forward, either she should be regularly taking birch control or you should be using a condom, or both.
I don't mean to appear to be coming down hard on you, but you could be in some serious trouble, and only time will tell.