Not all mice "look" pregnant, so you cannot tell by body shape/size alone. Mice with worms, intestinal blockage and just plain chubby can also look pregnant. Following are details from an excellent mouse forum:
http://www.thefunmouse.com/info/breeding.cfm#pregnancy
TFM has a section on sexing. This will help you determine her gender so erase all doubts of pregnancy if "she" turns out to be a "he". It will also help with sexing the bubs if she is in fact pregnant. The link is as follows:
http://www.thefunmouse.com/info/sexing.cfm
If she is pregnant, gestation is from 19 to 24 days with 20 the average. Towards the time of delivery a pregnant doe may stretch a lot and look uncomfortable. Also, all mice nest so if she does this, it means nothing; however, pregnant does will prepare a special nest so she would require a nesting box to keep her bubs safe and private. Try not to touch her tummy as this can injure/kill the bubs and harm her. For details on what to what to feed a pregnant doe during pregnancy (higher intake of protein and fat), how to prepare the nursery and care from birth through weaning, please see the following:
http://www.thefunmouse.com/info/breeding.cfm#dietnursing
http://www.thefunmouse.com/info/nesting.cfm
http://www.thefunmouse.com/info/breeding2.cfm
TFM also has an active/current forum with a section titled, "caring for mother and babies" where you can post questions. The link is as follows:
http://forum.thefunmouse.com/
Your mouse is very beautiful. I hope for your sake she is not pregnant and healthy.