First off: How long have you had them/How long have they been housed together? Since she was from a petstore, its possible that she was pregnant even before you bought her.
Here is a guide:
Breeding Guide: Determining Pregnancy
This is a great rat forum:
Goosemoose Pet Portal - Goosemoose Pet Portal - Index
The best recommendation I have is to move her into a 20 gallon + Aquarium. No levels. If/when she does have the babies, you need it to be baby proofed and safe so she doesn't give birth on a top level and they fall out. It will also protect her from getting pregnant again after she does have the babies.