Her name's Izzy. I've only had her like, a week. I got a second guinea pig, named Cocoa, a few days after I got the first one.
I thought that Cocoa was male (I'm positive Izzy is female), so my mom took him back to the pet store and had them check. I told her that they don't know a penis from a vagina, lol, but she didn't listen to me at all. They said he was female, but I still told my mom to take him to the vet on the way home to check. She said "Those people work at a pet store, I think they know the difference between a male and female." and she wouldn't take him to the vet.
I left him in the box we got him in for a few days to make sure he wasn't sick. I didn't have a second cage to put him in, so I really had no choice but to put them together (even though I thought he was male) because the box was getting dirty.

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Anyways, I looked at Izzy today and noticed that her tummy area looks a lot bigger than it did just a week ago. So I took Cocoa out of the cage, and I'm 99% positive that he's NOT a female.
But I can't tell if she's pregnant, or if she's just growing (because she's still young). She's also been eating more lately, too. Before she would only eat lettuce. Now she's eating her pellets, hay, peppers, lettuce and she ate a little piece of apple last night. So maybe that's why she looks bigger... But I'm still worried.
These are the first guinea pigs I've had, and I really have no idea what to do if she IS pregnant.

I told my mom that she needs to call the vet to get an appointment to get Cocoa neutered. She laughed and said "They don't do that to guinea pigs."
:|
So I'm going to have to show her something online that says they can be. I'm not sure if he's big enough though... Because he's still young too.

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I'm moving to New Jersey when I'm 18, and I can't bring a million baby piggies with me. So I'm going to have to find a shelter or something to give them to when they're old enough.

But what am I supposed to do with Izzy if she is pregnant? Is there any specific diet she needs, or anything like that?

Well there is no way to definitely know unless a vet does it. Your mother needs to be more supportive seriously. Find a small animal vet and call them and prove to your mom they do exist lol. And make an appt for both pigs. That's all I can say.
She may just be getting bigger, they grow pretty fast mine was double the size after almost 2 weeks.