I think my guinea pig is pregnant.

Hi there evryone! Exactly 8 days ago I decided to get myself a guinea pig and went to the pet store to buy one, I decided to get a female and over time probably get a male or another female because I was just starting out. At first she was a bit skittish and timid but i've read that this is normal. After 3 days she began to allow me to hold her, feed her, play with her and everything was fine. About 3 days ago i'v noticed she doesnt like leaving her cage and she doesnt like when I hold her at all. She has gained weight buying her and she has been drinking alot more water n urinating alot. Upon buying he I wasnt informed of her correct age but it could be observed she was over 3 weeks old. Knowing this I fear she maybe pregnant because she was in habitat with 3 male guinea pigs when I bought her and at tht age she is able to get pregnant.The pic of her on my bed is when I got her and the other was yesterday. Yu can see that she has gained a few pound in the 8 days but is pregnancy the cause for her excessive eating and drinking or her skittish behaviour. HELP PLZ!