Hi there,
First of all, what makes you suspect she's pregnant? Has she ever been housed with males/when was the last time she was?
Secondly, guinea pig gestation is usually 10 weeks long. You may hear the pups teeth gnashing about a week before she delivers, and you can also check to see if the pelvic bones are separating, which usually happens about 48 hours before delivery, but the pups will come when they want to, so everything is just an estimative guide. While she's in her cage, have someone keep her still while from behind, you place your finger just above her genitals, and press lightly. If she's not been pregnant long, you won't feel that for a while.
Thirdly, can you take a picture of her from above (without lifting her) to upload? Some pigs are just naturally pear shaped and many assume it's down to pregnancy.