The “Morning after pill” or urgent contraception acts in a way that high doses of estrogen hormones are given once in the first 72 hours after the sex. Blood level of the estrogen hormones rapidly increases, and because there is only one dose, that estrogen level will also rapidly fall. That rapid decreasing of the estrogen level should cause endometrial desquamation and bleeding (some kind of period). Without endometrium, the fertilized egg-cell won’t have a place where it can embed. Urgent contraception has an efficiency of over 90%. If your girlfriend got a period a few days after taking the “morning after pill”, her cycle will be disturbed but she will probably avoid the pregnancy. If she didn’t get any period you will have to wait until her next regular period comes. If the regular period doesn’t come she can do a pregnancy test.