Thank you for asking such an interesting question, which many patients of mine have also asked me at the clinic.
From the way that you described, it would appear that the chances of getting your girlfriend pregnant may be low but it would hard to be certain. A better option is to consider the use of a condom or other forms of contraception. You and your partner can discuss this issue with your family physician.
There are two components of ejaculation. One is the expulsion of the ejaculate (which contains sperms and various fluids). The other component is the mental sensation of orgasm. Sometimes, the guy may not experience the orgasm feeling but the ejaculate could have already come out. Hence the need to consider other forms of contraception.
Also, frequent interruption in the way that you describe (coitus interruptus) may also diminish the enjoyment that you can derive from the intimacy and close relationship. Furthermore, both you and your partner become anxious after the sexual encounter. All in all, it would be good to consider other forms of contraception and getting both you and your partner's anxiety resolved.
Take care and be happy!
