I just read that SOMETIMES the amniotic sac can re-seal! From an obstetrics journal abstract:

Two cases (27 and 23 weeks of gestation) diagnosed as high-leak PROM by an intra-amniotic dye (PSP) injection method later proved negative on re-testing by the same method, and ended in spontaneous reseal of the membranes. The pregnancies in these cases extended to 36 and 40 weeks of gestation without any eventful signs of infection.
Also read on a few other sites that this can happen. I reckon it did to me!! (I had a slow leak with Flynn's birth so I am pretty sure it was leaking earlier on...)