Congratulations on becoming an adoptive guinea mommy, it sounds as though your little girl is about ready to give birth given her measurements and the movement of the little ones.
Guinea Pigs tend to give a little hiccupping sound, and that seems to be an indication that the babies will be born a few minutes later.
She is likely to give birth early in the morning or early evening so it is best to keep a check on her at these times.
Labour should be quite short luckily, (usually between 15 and 40 minutes, depending how many little ones she has) and it's best to have the number of a vet nearby just in case there are complications but there is no reason to think that there would be.
Just make sure she has somewhere warm and sheltered to give birth, there should be very little blood and the babies will come very close together.
If she is still cleaning one when the next one arrives, you may need to help her by wiping it with a soft towel and freeing it from the protective sac.
Enjoy your new little family :-)