This could be very serious, and there's a 33% chance that this could be an ectopic pregnancy. The tie was enough to prevent the egg going to the uterus, but didn't prevent sperm from going up to see the egg, the fertilized egg implants itself in the tubes, but can only grow a short time before the tubes burst, and this has very grave consequences on the women. So get this checked out, and this link has some symptoms of ectopic pregnancies.
Ectopic pregnancy - Wikipedia, the free encyclopedia