Hi,
Sorry, I have not understood everything comprehensively. Please, do elaborate on what happened.
1. You ejaculated in your underwear.
2. You had oral sex. Did she suck you or you her?
3. You washed your trouser snake with water. Had you ejaculated a second time or why did you wash it?
4. Finally, you penetrated her vagina without a condom. Did you ejaculate insider her (a third time in total that evening)?
If you did, the chance is significant but low. The density of your sperms in your third ejaculate will have been not very high. Hence, chances of any of them having reaching her oocyte have plummeted.
If you did not, how should you have managed to impregnate her? You did not discharge inside her. All the sperms of your first (and possibly second) ejaculate will not have survived that long. Stay relaxed. Yes, a sperm or two may have remained inside your urethra and then been ejected into her vagina, but the likelihood of that sperm having later hit an oocyte approaches almost zero...
...unless you had deposited a third load inside your girl's vulva (see above).
Why does she not seek free help from the next NHS sex health clinic today? They are there for such purposes. 😉
Better luck next time. 😃