Female pigs are usually able to become pregnant at about four weeks of age. If we assume that she was in with boars at 4 weeks of age, and got pregnant right away, then she is now two months pregnant. Gestation is 59-73 days, which means she could possibly give birth any time from the next few hours to the next two weeks. You have to read as much as you can about pregnancy and birth. The link that PinkGrass gave you is excellent but pay special attention to the labour section too (link on top left of reproduction page produced by PinkGrass's link). Pay special attention for signs of pregnancy toxemia:
The sow stops eating, becomes depressed, and adopts a hunched, ruffled appearance. She may salivate profusely and the characteristic smell of ketones can be detected on her breath as she becomes ketoacidotic. The condition progresses to muscle spasms and death. The signs often occur abruptly. Pregnancy toxemia is more prevalent in hot weather. (This info is on the link PinkGrass gave you). I think ketones smell like nail polish remover or pear drops, but I am not totally sure. If your pig seems unwell at all get her to a vet asap, especially when pregnant. She is very young to give birth, but the better prepared you are, and the more you learn about what to expect, the better chance you have of helping her if she needs you. She may give birth without a hitch, but the web page will also tell you about the problems she might have. The fact that she is moving around well is also good. Many people put food and water at opposite ends of the cage to ensure their pig has to move about, as it is dangerous for a pregnant pig to become inactive. If you can get Cavy Performance pellets, made by Oxbow, introduce these to your pig's diet slowly, (until you are sure she likes them enough for you to stop providing the normal pellets,) as they are the best food for pregnant mothers and babies. If you can't get this food, try to get some alfalfa hay and mix it in with her normal hay. It contains calcium which she needs. There are lots of people on this forum who can help you with any questions about your pregnant piggy, and advise you about what to do about the pet shop. I would take your pig to a vet who has a lot of guinea pig experience to make sure that she is actually pregnant and isn't suffering from something else. You said she was dragging her back legs, and I think it would be worth getting this checked out. If the vet says she is pregnant I would go to the pet shop and tell them that your pig is pregnant, and that she got pregnant in their shop, tell them to seperate the boars and sows, not to sell any sows to the public until 74 days have passed, and that you expect them to pay for all your immediate vet care for the pig and babies. Goodness knows how many other pregnant pigs they have sold. Make sure at least that they know to seperate the sexes, and that they can breed from 3 weeks of age.
What kind of cage do you have, as you will probably need to babyproof it, and at some point make a bigger one. It would also be good to try and figure out what you are going to do about the babies. If you can't keep them, do you know anyone that you trust to take care of them as their own pets? People will come across this post soon and offer help.
I would get your pig to the vet to make sure everything is ok.