They don't "look" pregnant until right before giving birth (maybe a week or two before, I don't remember). She may begin nesting but there aren't any dramatic changes in temperament besides that (I'm sure there are some subtle behavioral changes but since you just got her they would probably go un-noticed).
Honestly, though, I would just take her to the vet. They can help more than someone over the Internet

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Are both degus females? If not, plan to move the male as she nears her due date as degus can mate right after the female gives birth.
I'm assuming she was with males in the store? The store should be able to tell you when she was with them, right? That will make estimating her due date easier. Are the males related? I would be very concerned about in-breeding.