My boyfriends mom had a pregnant guinea pig, she had her babies a month ago. So cute!
Anyways, she got really big, and I mean BIG.
What my boyfriends mom was telling me that, when a human is pregnant the baby slows down when it gets close to the birth, so you don't feel kicks or anything.
What we both assumed, as this was the case with Bandent (the pregnant momma), when the babies didn't kick or moved around, we thought perhaps guinea pigs are like humans, and once it's close to the birth they slow down.
I'm not sure if this is exactly true, but thats what happened with Bandent. About 2 weeks after the babies started showing little moment as the days went by, she finally had her babies.
I'd say give her a week or so, perhaps 2 weeks. If she is really big, and there isn't much movement I'd say she's about ready to pop.
Of course I'm no expert, so I would suggest taking her to the vet to get an expert opionion.
I'm only explaing my own experience.
Hope I at least helped a little.
=D