I'm a little confused.
A spay (ovariohysterectomy) is the surgical proceedure of removing the ovaries, oviducts, and uterus. There is no way a sow can be pregnant after being spayed, they would have aborted the fetus.
An ovarian cyst wouldn't be located outside the body, but I am a little converned about what you may be feeling. I'm not saying she may not be pregnant, or that she has a cyst or whatever. I'm only a first year vet tech student and even after education would loose my job for diagnosing something such as that.
I think you need to get your parents involved, that's very important. Do they know you're expecting pups out of your girl? Serious complications can occur so its important you be truthful and honest to your parents so that your girl can get the proper care she needs throughout this pregnancy and so that her cage mates can be neutered! I would also highly recommend taking your girl to a DIFFERENT veterinarian for a second opinion. I think some tests need to be done to determine exactly what is going on here. I'm not sure if you were just confused by what the vet said? A good link was provided to help you find a
cavy-savvy vet.
Good luck.
I also hope that from now on you can be open and honest with us, there is no reason not you. Sometimes mistakes happen, we're all at fault of that at one point or another, it's realizing that mistake and how we handle them that really matters. Your here looking for assistance I'm assuming, and we can't properly help you without the truth.