it's still possible. you don't know what rats sold before her as feeders or as pets.. rats can come and go fairly fast in pet stores. and unless is was petsmart where they don't sell feeder rats and only offer ONE gender (even then sometimes they'll get a rat of opposite gender in by mistake) then I highly doubt the place was good enough to seperate genders... or the place the girl came from before she even ARRIVED at the pet store (mass breeders don't seperate obviously).
they can get pregnant as young as 5-6 weeks.... in one case a rat got pregnant at 3 1/2 weeks old, by her father!!!!
if she is pregnant, offer more PROTEIN in her diet!!! cat food, chicken bones, bits of meat, etc.
good luck.. rats can have as many as 20 babies in a litter. I know someone from my rat board who's two girl rats got pregnant from her friend rat-sitting while she was away on vacation (I believe it was vacation?). she wound up with two litters of 17 babies each!!!! 34 babies on top of the rats she had, LOL! good luck. they truly are worse than rabbits. just not as bad as mice *coughs*
2 rats, 2 mini rex bunnies, 2 cats, 1 prairie dog, 4 species breeding millipedes, 23 various different species tarantulas, 2 Heteropoda davidbowie huntsman spiders, 3 Liocheles australasiae scorps, 20+ Hogna helluo wolf spiderlings, etc.
spiders are addictive... and super easy to care for!