Pregnant????

Help!!! My female guinea pig has gotten considerably larger within the past month from her mid-section to her rear. My other guinea pig is a female or at least that's what I was told and thought, but now I'm not so sure. Could someone please tell me some signs of a pregnant guinea pig!!....She hasn't been acting the same either....I've also looked at websites and I haven't found one that seems to show what the signs of pregnancy are.....Or could she have gained a significant amount of weight to make her look like this even though there has been no change in her diet??
Please Help Me
Very Concerned,
HoneyandDutchess