go here and ask for help: http://www.cavycompendium.com/forums
They have a pregnancy section, just like here.
Guinea Pigs are pregnant about 72 days, if you can see the babies moving she probably has between 2 weeks to right now before the babies will be born.
I'm guessing you've got about a week or so to go, unless the humane society was dumb and put her in with a male.
She doesn't need to be separated from females, unless they are fighting. Other females have been known to help out the moms by cleaning the babies and keeping them warm.
I wouldn't feed her lettuce, unless it's something more substancial like Kale or Spinnach. Iceberg lettuce is pretty much just water and can cause diarhea if they eat a whole bunch.
If she is over a year old and has never had babies before, there is a chance that her hip bones have fused together. If so and the babies are too big, she will need a c-section or she and the babies will die. However, because it sounds like she was dropped off pregnant, i would guess she's either had babies before or she's under a year of age. And not all sow's hip bones completely fuse.
As for the babies, you can handle them the moment they're born! I recommend frequent short handlings to get them used to you.
For right now, just see that she's comfortable with continious access to water, pellets and alfalfa hay. Mom's and babies need the extra fat from alfalfa hay. When she has weaned them, you can switch her back to timothy hay. the babies can eat alfalfa until they are about 6 months old.
Keep her environment as stress free as possible. I would also recommend not picking her up right now unless you absolutely have to, as pregnant sows can double in weight and picking them up can cause stress on their backs.
Feed her lots of fresh veggies, red bell peppers are the best, having a high concentration of Vit. C.
But mainly, enjoy her! they are so amazing when they are pregnant. They have one of the longest gestation periods of any small animal, the babies will be born with all their fur, their eyes and ears open and running around.
Even if you can't afford a vet, look around for a qualified small animal or exotics vet, just in case.
Let us know when she has them, and post pictures! Baby "wheeters" (my name for my heard of guinea pigs) are the cutest animals you will ever see.
So, I hope that helped and wasn't overwhelming. Definitely go check out the site above and read through the forums. There are some really experienced folks on there who can answer just about any question you may have.