My children got guinnea pigs for Christmas. The pet store said they were both females but now I am starting to wonder. They should both be around 9-11 weeks old

I am pretty much their main caretake which is fine with me because I absolutely adore them. I have been watching their behaviors and the abyssian (sp?) one which is the bigger of the two always lays over the other smaller, straight haired one (like a cross). Plus there is always the rumbling noises that we hear a lot.

I read that sometime females will do that to other females to show their dominance but then today they were acting peculiar and I decided to look at their genitals. They definately don't look the same. I also looked on the computer at a boar and at a sow and both of those looked just like both of my guinnea pigs.

What are the first signs (symptoms) that a pregnant guinnea pig have?

Is there something I should do right now even before I know if she is in fact pregnant?

Thank you in advance...you will probably be hearing more from me especially if she is pregnant...I will need some help, I am sure!