I wonder if a lot of people just aren't aware of how early they can breed. I know I had no idea until just recently when I started thinking about getting a guinea pig and doing a little reading online. Most animals aren't even weaned from their mothers at 4 wks old, much less breeding. If you didn't know guinea pigs and just got a couple, you might think you have a while before they're breeding age, like kittens or puppies. You wouldn't think twice about two 8 wk old intact puppies being together, you know?
Unfortunately the people you get them from, unless it's a shelter, don't educate you on this. This is my 3rd time having guinea pigs, and I'm just now finding out how early they can breed. I was just a kid when I had the others, the first one was just one male, and then I had 3 females at once, luckily none of them were pregnant when we got them. I'm considering myself new at this though, since back then we just fed them pellets and a little lettuce and carrot here and there, their cages weren't up to the standards we know about today.
I'm sorry, didn't mean to ramble. I'm just saying it's easy to make mistakes with such a unique animal when you're new at it.
Fearless, I hope your guinea pig is okay.