Although it is unlikely because of what menstruation means (the releasing of the egg and starting a new cycle) it is still possible to impregnate the egg before it has a chance to leave.
If you want to know if she is pregnant, you'll have to have some patience. She won't know for a week at least, maybe a couple. There is a hormone found in excess in your body when you become pregnant that the pregnancy test picks up on, you will have to allow time for that hormone to get through her system so she can take an accurate test. One to three weeks is about all she should need. There are other symptoms as well, but most won't become relevant until after the time frame you are waiting for to do the test.
Hope this helps :)