Pregnant rat?

[aliasrestricted]

   About two weeks ago I picked up two female rats from a pet store, I've got to say I love them by the way-- amazing pets. Anyways over the past day or so one of the two has been exhibiting some peculiar behaviors. I lined the bottom of their cage, underneath the bedding, with some newspaper which they loved to chew into strips and stuff into a little denim tunnel I made for them, and within the last day the one in particular has essentially erected a paper fort in one of the corners.
   She has also seemed to have gotten larger, but I'm unsure of how much; her belly seems much larger than her cage mates and she has been eating near constantly when awake or stashing food and any trinkets she can find in her fort. I haven't yet been able to find any nipples, though she is reluctant to let me look.

Any opinions?

[Kiko]

Be wary of Megacolan, a fatal and painful disease that makes rat appear fat.
Can you provide pics?

[Soybean]

Pics would be best. Megacolon usually affects rats when they are younger (most not making it past weaning age, but I'm sure there are adults affected by it). If she is exhibiting nesting behavior (and that definitely sounds like it) and getting larger, then she is most likely pregnant. The appearance of nipples doesn't always mean pregnancy though. Look at her belly and sides. If it is starting to look like she swallowed two small baseballs, you have a pregnant rat. If you posted pictures (from above and from the back) I could tell you if she was pregnant or not.