Wow! Definitely looks pregnant and due very soon! A fat rat would be chubby all around but your rat is bulging on the sides, also this big is normally in the last week of pregnancy so get ready. Can you try to press the sides to see if there's anything moving? Also, where did you get her from?
I got her from Feeder’s Supply, and no I haven’t seen many maternal behaviors. I do see her nipples in that photo but only slightly. She hasn’t been distant either or aggressive at all, but I’ve seen her stash a lot of food and drinking a lot. I thought she was just chubby but I haven’t seen any other rats bellies jut out that far!
According to the rat guide, loss of hair around nipples and nest building normally occurs close to the last day of pregnancy. If she continues showing signs of pregnancy you can start adding some vegetables and grains to her food: https://www.petcoach.co/question/?id=411243