Is my mouse pregnant? help?

i am pretty sure she hasnt had contact with a male mouse but im not sure i let her play with my friends mouse 2 weeks a go but were pretty sure its a girl. anyway shes getting plump all around her body she looked like she swallowed a golf ball and her are big and ping and every time i give her nesting material she shreds them and shuts her self up in her itty bitty igloo all surrounded by shredded toilet paper and hay is she pregnant? help?