guinea pig possibly pregnant

Question
I got a female guinea pig 3 weeks ago, she is roughly 3-4 mnths old I have 4 other girl's aswell, I messaged the previous owner approximately 1 week after I got her to let her know that she was settling in well, then she mentioned she may be pregnant, as she was housed with boys before coming to me(apparently her whole life) don't think she's to far along if she is, since i've got her she's put 30grms on, what are the chances she is?

Answer
The chances are extremely high that she's already pregnant.  The good news is that even being young she will deliver without a problem.  

There's no way you can tell exactly when she will deliver but you can make an educated guess judging by when you feel babies moving. That's typically about two weeks prior to delivery. There's nothing different that you need to do for her other than keeping her cage clean and keep plenty of food and fresh water in front of her all the time.

Guinea pigs have a built in maternal instinct so you don't need to separate her from the other females unless they aren't getting along. The other sows will be good nannies to the babies and will help to take care of them. Some of them may even try to nurse the babies even though they're not pregnant or nursing already.

It's unfortunate the previous owner didn't share that information with you at the time you got her. That would have been the responsible thing to do.  But the bottom line is there is nothing you can do about that now. So just enjoy the new babies, they grow very fast.