My doctor actually told me that most HPT (home pregnancy tests) work 20 days after the egg has been fertilized but even that is not the same for everyone. Some women can't tell till much later. I myself couldn't tell with a HPT until I was 12 days late for my period.
Blood tests can be done 8 days after the egg was fertilized but again that's not 100%.
Some women don't produce much hCg (pregnancy) hormone and can continue to get a negative result until much MUCH later in the pregnancy. My mother couldn't tell until she was 6 months along even thought all the other symptoms were there including my sister kicking her.
Every pregnancy and every person is unique.
As for a preventive measure for the next time. Look up Plan B or the morning after pill. that's the only contraceptive that I know of that will work after intercourse. Mind you it's not going to help you now in this situation.
Look up all different birth controls and then talk to your girlfriend about going on one of them. The condom is only about 80% effective where as the pill is 99%. But remember nothing is 100% so if you're not ready for a baby then I would rethink having sexual intercourse all together. I know several women who have gotten pregnant on the pill, the patch, the shot, the pull out method (also known as the pull and pray), and even having their tubes tied and a vasectomy.
Good luck and I hope that this helps